What sealing does:
Helps protect - makes spills not fully penetrate into the surface.
Helps make clean up easier (food stains, grease, oil, tree droppings.)
Help slow down fading from the sun.
Some help assist in locking in sand joints.
Enhancement sealers make the surface color pop again.
Some have algae and mold inhibitors to help slow down the growth.
What sealing does not do:
Does not make them slippery, if using quality water based sealer.
Completely prevent mold and algae (you cannot beat Mother Nature.)
Does not last forever.
On average needs to be done every three years.
Give an absolute finished look.
The biggest part to how a surface will look when finished is how porous the pavers are.