I'm not new to car detailing and have always washed/clayed/polished/waxed my cars at least twice a year.

I've been looking at wax sealants but I can't for the life of me find out what they actually do.

I've always waxed at the last step in detailing up until now which I thought would protect the surface.

I've always thought:

Wash - Get rid of loose dirt/dust.
Clay - Remove stuck-on contaminants.
Polish - Add shine
Wax - Seal in shine
Sealant - ? (what does this do that more wax layers couldn't?)