You can safely set slices to 1 for everything. Only Blu-ray level 4.1 requires 4 slices.
Each slice divides the screen into a completely independent piece. As such, more slices can reduce the ability of the encoder to compress a file efficiently.
The reason for slices is to speed up decoding. Each slice is completely independent and so each can be decoded independently. This is why Blu-ray spec'd slices for level 4.1. It is cheaper and easier to implement four simple hardware decoders than one faster more complex decoder. Maybe not so much now, but when Blu-ray was being dreamed up it was more of a concern.
Exactly what software decoders on computers do with slices is unknown to me. Youtube/Vimeo re-encode our uploads without slices.