About pre-purchasing game on Steam

No, you gain NOTHING by pre-purchasing a Steam game. No advantage at all.

I do not recommend any pre-purchases at all.

no.
sometimes you get some skins, addons and stuff for free

as for Mighty No. 9 you can see on the storepage you will recive a free dlc

Only if it specifically says so. For example, The Witcher 3 was 10% cheaper on pre-order, but it was clearly mentioned that it's 10% off. If the game is not marked as a "sale", it's not cheaper. Mighty No. 9 has no such discount. Even if it did, I'd think twice before paying in advance for a game that may or may not be terrible.

Unless there is no pre-order bonus, there are exactly 0 reasons to preorder. Just look at the sh*t about Arkham Knight's release. Believe me, at the state of the games (or at least vaaast majority of them) are coming out nowadays, you don't want to pay a single dime to the publishers before you know it will be good and it will be working.

View attached image.

Maybe you should watch this, there are some interesting point listed.

Well, on Steam it can come with a -10% discount, on GreenManGaming it sometimes can go as high as 30+% for certain titles. But it's been long since I've seen digital pre-order bonuses. Publishers tend to put pre-order extras into box editions in the form of utterly useless plastic toys and cases, usually.

Never pre-order anything.

sometimes there is special items or other games in pr-purchase deals
like the bureau had all previous xcom games and spec ops
that was worth it but if you can wait few months the game comes on sale and then you should buy it :)

Buy it when it comes out that is the best or trade it

I only pre-purchased a game once and i had no problems on release and i even had early acess to it. But i would say thats luck. However, its usually better to wait instead of getting games that dont work correctly.

I "pre-purchased" some games, because I supported them on Kickstarter... dont know if this counts... There are some really nice ideas. And why not support them?!

Well, against some other commenters here, I am of the opposite opinion, namely that there's no reason NOT to pre-order in your case:
If you preorder, you get a free DLC, which in your case could be called a discount or a nice freebie, depending on your will to buy the DLC if it were not free. Assuming it is a disappointment on release, it should not be a problem, just get a refund! (but don't play for more than 2 hrs before deciding, otherwise it's way harder to get a refund). If you think about preordering it, probability is big you will like it anyway - then it is good to have preordered. If you don't, you didn't really loose anything either.

I only pre-ordered GTA5, that was the 1st and probably the only time I pre-ordered.

Its depends on game. You can do that on game like GTA5. Becouse you know it will be good. Or Fallout 4.

sometimes the bonuses are definitely worth it. not often, but sometimes. remember bioshock infinite? chances were excellent that it would be a good game anyway. and i got 3 free games, i think (including XCom EU, which was not that old at the time), and 30% cashback (blue coins). that was a good deal, and preordering was definitely worth it. i admit, most preorders are garbage (the ones were you get 2 skins...). but while all scream "no preorders" again and again, like parrots, it should be mentioned that there are decent deals from time to time. :)

That game will probably fall fast in price, also it's cheaper buying it somewhere else than Steam, like Nuuvem, or other official resellers.

I would wait for price fall, but that's me.

I pre-ordered Torchlight 2 because it had the Mac icon next to it...
It turned out that they only released the PC version first and Mac users had to wait for many years -_-; And the price went down like 75% on sale... and end up on HB later (And there was maybe 10-20% discount on release date? don't remember)

Anyway, I'm against pre-order... Unless if you really want to try out the DLC asap...

You get just the retro hero DLC for free.
I'm pretty sure it's cosmetic, but I'm also pretty sure you'll be able to buy the game and the dlc for less than the pre-purchase price.

Imo it's not worth.

i'd like to point out that the game is cheaper on nuuvem if i'm not mistaken

Depends on the game. Witcher 3 for instance had a 10% off preorder price. Some games have extra stuff like skins or weapons or other dlc, and some games have nothing. If it's the last one, then no reason to preorder.

Preordering digital games is basically pointless (unless there's some kind of discount/bonus), don't let developers suck your wallet off from hype alone.