When these lands animate, they become creatures—and creatures, the philosophy goes, should have creature types.
Forbidding Watchtower, as a fortress full of soldiers, gains the creature type Soldier, and can reap benefits from Mobilization and Field Marshal. Faerie Conclave, logically enough, becomes a Faerie—finally a reasonable card to include in your Faerie Noble deck. Spawning Pool spawns a Skeleton, which matches Magic's mechanical pattern of so many 1/1 black regenerating creatures being Skeletons, and might matter if you have a Coat of Arms in play and are building a Drudge Spell army. Ghitu Encampment, as the base camp of the rugged mountain warriors known as the Ghitu, becomes a Warrior—pumpable with Lovisa Coldeyes and a reasonably powerful combo with Sosuke, Son of Seshiro.
So, why does Treetop Village become an Ape? Because the card depicts Yavimaya, a forest and valley where a race of sentient apes once made their home, and because the first printing of Treetop Village showed these apes inhabiting the village back in Urza's Legacy. (See also cards like Gorilla Pack or Yavimaya Wurm for more flavor text relevant to these apes.)
Tenth Edition Treetop Village art by Rob Alexander
There's another noncreature card in Tenth Edition that gains a creature type when it activates: Chimeric Staff, which becomes a Construct when you pour mana into it.