I'd say there are two reasons:
1. Some translations are actually pretty bad. I mean, I wouldn't mind finally rereading Fate if it actually got a new translation. The problem here is that it never will, and if it does, it won't be done by MG.
2. Some people just want a way to get a way to support a novel they like officially. Not everyone is comfortable buying games on Japanese websites. Especially since many games don't even have download editions, so you have to import a physical copy, which leads to many different problems. And some older games, especially by smaller publishers, are often not available at all. I tried to look for a way to buy Rose Guns Days, but I was out of luck.