Using split cakes as a beginner.

What is a practical use of leftover paint from split cakes that have worn down unevenly? Also, does it get better as I become a better painter?

you could carefully cut each color apart and remake the same split, or a new color combination, by adding colors from other splits or solid cakes.

or use the separate colors for practicing.

or make little pots to use for double dipped flowers or dots.

all cakes, including solids, wear in the middle first, because you tend to press the brush or sponge there, where there is room.