Not all licenses allow charging for forks. You can charge for your services always. And you can charge for code that is all your own. But, only certain licenses allow you to actually fork and charge for it without sharing those contributions. And many might not even really consider those licenses to really be FOSS.

It's particularly popular for startups to use to bootstrap their tech company and build cred shortly before they reach the "we have to actually turn a profit" phase, at which point the bean counters try to squeeze every bit for a nickel. Once they have marketshare, they say, "we are helping the competition by releasing this!" and abandon the things they actively maintain.

There is also a direct benefit for open sourcing: you can get other people to debug and improve your software for free. They go the enclosure direction once they want to squeeze their customers for more money, e.g. closing the source code and charging $x per use of the software to their service clients.

Once they're a monopoly, companies can swing back to the open source direction because they have no competitors to worry about and can just get free dev work and good will out of it.