The family runs a modded Minecraft server, and picking a world seed for it used to mean actually generating the world and rendering a map to look at it, which took about nine minutes per seed. Nine minutes does not sound like much until you want to look at a dozen seeds and compare them, and at that point nobody did it, because who is going to sit through an hour and a half of world generation just to browse. So we picked whatever seed looked fine on the first try and lived with it.
I wanted to be able to type a seed and see the terrain right away, the same way the vanilla seed map sites let you do for unmodded worlds. Ours is modded though, so those sites are useless to us, they have no idea what our biome tables look like.