I'm not a designer of any online game, so I can't talk for them, but considering experience and human behaviour, plus having played some of the games from gameforge, the idea is very simple.
Captive market to make money. Either by selling in game products or by promoting stuff.
They have captivating games that interest people using different techniques, so they have than once you are inside the system you are going to be hooked to the game, at least a good number of people, they don't know for how long,but it's going to be some time for sure.
Part of those games involve things that you can get with money, improving response from the game, faster development, many things that players desire, and some of the registered player will pay for that. The percentage may be low, but there is always some people that will, so the most people that you can get inside, the better.
Other games, and also gameforge for some time, send you newsletters with promotions, and they may have deals with other companies to get their products promoted. Since they already have a big amount of subscribers who are not going to block mails from the game they are playing, that guaranties a lot of eyes.
Of course there may be other reasons more or less legal or moral that we don't know about, but that's even more speculative.
Should you do that? I can't tell.
If your interest is to make money and you can give users something they want, sure, go for it. If you can't give users something worth the effort and/or the money, then don't.