Just like it costs money to host a website on a server, it also costs money to host a server for a game, and then then you have to make sure the server has decent latency for gaming.
You can look at an example of this with GameSpy where the company was losing money due to lack of funds and had to shut the service down.
https://www.engadget.com/2012/12/14/gamespy-multiplayer-server-shutdown-result-of-failure-to-pay-by/
Sony on their introduction of paying for online essentially states the same thing.
https://www.destructoid.com/sony-explains-why-you-ll-have-to-pay-for-online-with-ps4-256158.phtml
So because they don't have to shell out cash for the service entirely, it allows them to give us more features with online at that. There's essentially nothing wrong with payed online, you're paying for something that costs the company money to host.
You can look at an example of this with GameSpy where the company was losing money due to lack of funds and had to shut the service down.
https://www.engadget.com/2012/12/14/gamespy-multiplayer-server-shutdown-result-of-failure-to-pay-by/
GameSpy Technologies has also responded to the issue on its Facebook page, and says that the server shutdowns are a result of the game developers choosing not to pay their service fees, and that "in some cases this lapsing ranges back as much as four years."
GameSpy Technologies tells players that it "cannot be expected to provide a service free of charge," and that's why these servers went down.
Sony on their introduction of paying for online essentially states the same thing.
https://www.destructoid.com/sony-explains-why-you-ll-have-to-pay-for-online-with-ps4-256158.phtml
After being asked what went into the decision to charge for online gameplay, Sony explained that online features and services are going to be a "big pillar" of the PS4 experience. They go on to note, "we've already talked about the share button, sharing features, second screens...if we continue to invest into online infrastructures, that costs money. If we continue free service for online multiplayer, from a business standpoint, there's pressure to recover costs.
That's when we decided 'let's ask the people who are most active in playing online to share some of the cost so we can invest more.'" It has also been noted that some F2P PS4 games will not require PlayStation Plus, including Planetside 2 and DC Universe Online (the latter of which will feature cross-platform play).
So because they don't have to shell out cash for the service entirely, it allows them to give us more features with online at that. There's essentially nothing wrong with payed online, you're paying for something that costs the company money to host.