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From people who would rather pay for extra space rather than split their files between multiple free accounts.

Businesses and people who either need it all on one account, or are too lazy to make a second account. Also, ads.

Google have a lot of other ways to make money

Other services have premium accounts and ads

They print it? That is how they make it, maybe?
I think that is where money comes from.
Not really sure anymore.

Ads.
Subscriptions to get rid of the adds.
Subscriptions to allow larger storage on same account.

If we're moving into shady territory:
Reflink adds for porn

If we're moving into more shady territory:
Selling your email address which you probably used to sign up, along with your other personal data

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good luck ninjas

They also use file deduplication which means that a lot of that space costs them nothing. The way it works is that they only keep one copy of each unique file across all users and just share the access to it. Since for most home users, truly unique files such as personal documents and such tend to be smaller, and the larger files they might store such as music, movies, programs, etc would be identical across most users, they can appear to be storing a ton of data in the aggregrate while actually using only relatively little.

Since paying accounts are just as likely to fit this profile, or perhaps even more so, since common multimedia content is often what takes up most HD space for most people, and they're still paying for all this apparent space, this lets the company have very high margins.

In fact, the more customers, the higher the margins can get, as they're more likely to have repeat use of even hard-to-find files. It also makes the sharing features specially ludicrous, as they count the space used for every account that has share access to the file, despite only storing one copy.

Of course, this also means that when they claim the files are safely encrypted, it is only a technical truth. Yes, the files are encrypted, but they have the key, otherwise they couldn't use these methods. Since they have the encryption key, then the encryption isn't really doing anything in the first place.

Megaupload worked the same way, btw.

Google doesn't make money from it, but they just want to have every kind of service so people will only use their other stuff and profit there