Who is it that puts real money into the TF2 economy, and how do they profit?

Basically people who are selling the keys for really low, got them basically for free by trading metal for those keys. Metal that they made by scrapping common weapons in the weekly drops.
There are people that get the keys to open common boxes, some of them get cheap keys to trade for games and other stuff.

2 weapons = 1 scrap
3 scrap = 1 reclaimed
3 reclaimed = 1 refined
3 refined (generally) = 1 key

If you see someone asking say 1.55 ref for something they want...
1 refined (1), 1 reclaimed (.33), and 2 scrap (.22)

And no I have not put a single dime into the game, but I have gotten lots of stuff. My first week playing, I lucked into getting a salvaged box. It was going for 6 keys at the time. Now it can be traded for like 15 to 20 keys or something like that.

Eastern Europe and Western Asia can obtain TF2 items for a significantly cheaper price (especially Russia), if I recall correctly.

Russians with stolen credit cards bring a lot of the keys into the market and sell them for under cost.

There are people who use the keys to gamble as certain crates can give items worth many keys in value. Other people use unusual hats as something to invest in as they're usually $30+ and then they just try to flip it for more. If you have any type of day job you'll make way more with your time than either of these options.

IDK, but I just sold 6 keys that I got from a trade on the community market @ $2 (+$0.30 valve tax) a pop, so whoever it is: thank you

Keys are used as a common currency in trades. Traded for games/game keys/ services/ and other game items like Dota 2 and TF2 items including weapons and hats.

You can take it as an investing or to buy other games for a cheaper price.

I can make $30-$60 daily trading items in tf2, so it's not a bad invest if you got time to trade.

I'm guilty of dumping $50 for a polycount pack way back when, about $100 total into tf2 back when I played it a lot.
I don't really regret it since I bad my fun with friends and whatnot, but those $100 could have gone a long way. xP

I found an old thread that explains the economics behind it. Prices are different, and the Community Market didn't exist then, but it gives you a general idea.

f.e. if people buy games worth 10€ with 75% sale on it, it cost 2,50€ for them, 3 weeks later they sell it for 4 keys to someone else, this guy only paid 8€(mostly even cheaper, steammarket price for keys are around 1,75-->41,75=7 euro) they will spare some money and those "games on sale buyers" sell those keys for 1,3€ each again--->1,34=5,20, so they will also make some profit. russian guys can sell games way cheaper than western EU guys, cause their store price is lower than the ones from US or EU

A fairly big portion of the keys in circulation comes from people with stolen credit card information that buy a bunch of keys, trade those keys for buds and then sell those buds for money.

And no Valve either doesn't care or has decided that removing items that can be traced to a fraudulent purchase would cost them too many TF2 players.

Who buys them? - Basically anybody who needs to rise capital in tf2. You can go and buy any item or even a game with it.

Why is the market value lower? - It's not because of cheaper regions, well, atleast not directly. The way of getting a key for a lower price is when you buy a game on Steam and then you sell it on the market for tf2 keys. You can lower the price of keys by 20-40% if you use this way. And the "russian effect" works here too. You cannot buy tf2 keys from Valve for a lower price in different regions, but you can buy the games cheaper. That's why poeple from these regions usually resell games, they can get a key for $0.8-$1 by this way. They usually sell them on open markets for the mentioned price $1.3 (it's probably a bit higher now, the value of keys has been rising pretty quickly recently, I'd say they're more like $1.5-$1.8 now). Or they sell them on the Steam market more often now, it's more comfortable way for both parties.

Why are they keys so popular? - The tf2 key is the most liquid item, you can buy with it basically anything or you can't sell it quicky for a good price anytime. They hold the value well too, they went up by 30 % in the last 1-2 months! It must sucks for those people that held capital in metal...

Who are the end users of the keys? - Yep, those guys that open crates. It's just another lottery. You can't usually make a profit with most of the crates, and if you can, they're quite rare and expensive (eg. crate 19, or the salvaged series). But there's always a 1 % chance of getting an usual hat. And that's the jackpot! An unsual hat can be worth from $30 up to $1200.

I always assumed it was people with alt accounts who bought 2 keys to make premium with their $5 in steam wallet, and then took a slight hit in trade.

I'm fairly sure that keys in the russian store are or at least used to be $1.

unlock a rare weapon? what? UNLOCK AN UNUSUALLLLLL obviously.
there are plenty of people who buy them

There are some reasons why keys were cheap before (the era they were 2 refined at most) take it like it was a huge black market of items. You could get unlimited items via idling with no cap before tf2 went f2p(cap 12 items maximum per week). Mostly by russians mafia :D

After that keys price a little bit at were 2.66 max for a year then unusual halloween themed were introduced in 2011 and since those have 1% to get in limited time and crates,specially expensive. In 2012 that totally broke keys marketing since everyone want those unusual that are valued hundred of dollars, key hoarding before halloween was the main idea for the event and there was a huge demand for keys.

And it got better with the introduction with the marketplace(sarcasm) now they are reaching 4 refs each key and thats why those are being sold for 2 dollars each(and people buy those).

Its too damn easy to get extra metal to buy keys with several accounts and guess what, russians have the ability to deposit any amount of money in their steam wallet, the cheapest item in mann co store is 50 cents. Russians can make 10 idle accounts with 5 dollars unlike the rest of the countries are obliged to deposit 5 dollars minimum per account.

TL DR version = RUSSIANS
and TF2 is srs bsns

So...does anyone actually use the keys to open crates?

I'm so glad I'm not involved with the TF2 economy.

Normally people don't use the keys, they just keep em for trading for games or they trade it on for a higher price than they got them