you need to use the axiom of global choice to show that a category is equivalent to its skeleton, which can be pretty important. Basically the skeleton of a category removes all "unnecessary" isomorphic objects: if two objects in the skeleton are isomorphic, they are equal. (So cardinals are the skeleton of sets, ordinals of well-ordered sets, for vector spaces, you still have cardinals, corresponding to dimension, but different morphisms etc.)
You can construct that without choice, by modding out by isomorphisms basically, but you can't show that it is equivalent without choosing a represe…