Question

Why is it important to have cartilage in movable joints?

Answer 1

Without the cartilage, the bone would rub against bone, which is what happens in some case with people with arthritis. 
And without the cartilage, the bones may try to fuse together.

Answer 2

It builds new bone as the bones wear down. It keeps the bones from rubbing together. It holds the bones of the joint together. It connects the bones to surrounding muscles.