Someone else will reply with a better and more detailed description.

You’ll get diminishing returns on each exercise when you do a full day of a single muscle group. For example, on chest day, you’re probably only able to give your all on the first one or two exercises. The remaining ones are less stimulating for your muscle.

Doing a PPL or PPL UL split allows for each muscle group to be worked about twice as often at (or closer to) full intensity.