Because there is no theory of quantum gravity we have no idea how gravity could interact with anti matter. By showing that antimatter behaves just like matter when interacting with gravity we can learn a lot about it and cut the number of possible theories of quantum gravity in half.

Because things exist

(the antigravity hypothesis was an attempt to explain why matter and antimatter haven't annihilated each other)

Because one common assumption was that the universe might contain as much antimatter as matter.

Which begs the question: Where did it go? We would notice a huge amount of annihilation reactions in the solar system.

"Antimatter falls up" (is gravitationally repelled instead of attracted by normal matter) was an easy hypothesis to explain that.