It shows you the map for A Seed. Put seed in, see map of seed. Ask it for random seed, it generates random seed, then shows you the map for that seed.

It does that very well. It does exactly what it says on the tin, no more, no less.

There's no such thing as 'what kind of map to generate' or anything like that; all of the options are just filters / things to show on the map.

Map generator = shows where everything is on any given seed

It's not making the maps up or anything like that, all it's doing is taking the code you give it (or generating a random one) and showing you what that map looks like, so that you know where the trader is, where the bosses are, where the dungeons are, what biome is where, what the continents look like, that sort of thing.