That's my point. They don't have ads or anything remotely similar, so why bother with this then? It's annoying, so why use it as a promotional tool for people that already have the game?
Because it is in theory fun/time waster (honestly if I'm not specifically looking for a certain mon/area* I do kind of like going through and seeing what mons I get)? A neat (well, I think it's neat) way to get "new" Pokemon? A selling point for the game? And theoritically brings in revenue via people buying guides (see: Everyone here that imported guide books (sometimes multiple times!)), cards, or getting you to go to a store to get a code and perhaps buy more things.
I'm willing to bet when they partner up with other places they're paid for the team up a little. So like the Croagunk promotion should have theoretically brought you to IGN where you might do xyz
*I like that now that the mountain is out everyone's getting tons of Manors and are all "ughhh manor"