I think the name has a lot to do with it. If somebody is a fan of a series and a new game comes out in that series, they are likely to buy the game just based on its name even if they didn't pay attention to any reviews or hype or whatever. I buy and play Zelda games just because they are Zelda games. You could title the new Zelda something silly like "The Legend of Zelda: The Royal Tea Party" and I'd buy it. Of course, just because it's titled Zelda doesn't mean I'm going to like the game. This was the case with Twilight Princess, but I bought it because it was Zelda.
I think, though, it eventually stops working. I originally kept buying the Pokemon games, but after a while I tired of the series all together, and now it doesn't matter if a new one comes out- I don't buy it.
So, I think that it's an important part of marketing that helps make sales, but I don't think it'll work on everyone forever. People will eventually stop buying games that are part of a series they enjoy if the games themselves stop interesting them. Having an ongoing title helps and people definitely buy Mario games because they're Mario games, but it's not something that should be completely relied on.