To give your antagonist some good traits

1. Because it makes them more believable.
2. Because it gives them something for the audience to relate to.
3. Because it keeps them from becoming predictable.
4. Because otherwise they risk becoming dull characters.
5. Because it adds complexity to their character.
6. Because it makes it more likely for the protagonist to have something in common with the antagonist.
7. Because it can build trust between the antagonist and their minons.
8. Because it gives them the possibility of finding redemption.
9. Because it gives them something to confuse the protagonist with.
10. Because it is more challenging for the antagonist to balance both their good traits and their bad ones.