What Is It?
A Park Ball is basically a Master Ball in a different color. It has the same catch rate as a Master Ball, but can only be obtained by defeating a Boss Pokémon, while the Master Ball can be obtained by a Boss Pokémon or a Grotto.
What Pokemon Are Recommended To Use This On?
The Park Ball and the Master Ball have the same effect which therefore makes both very useful for catching legendary Pokemon or other rare and powerful Pokemon.
However since the Park Ball is rarer it would be some people's interest to use it on very special Pokemon to them.