There are a lot of companies out there who do not have a system administrator capable of setting up or maintaining (the latter is more difficult and time consuming) its own server. GitHub is a solution to that and is quite cheap.
When I checked github (I tried to check again now but their site is down) it would cost something like $1,500 bucks...It seems like I would be getting gouged.
The reason I say active is because even if you do have a thousand repos, you're surely not maintaining all of them - that would be insane. You would probably have 950 repos which would be better off archived, backed-up, and removed from GitHub.
Having said that, personally, I prefer BitBucket pricing scheme where I can have unlimited private repos and paying per number of users.