Question

Does sand spread on table increase or decrease the friction between an object on the table and its surface

Answer 1

Sand reduces the friction
It's particles are small and irregular and they are not attached to each other or anything, hence if we try to move an object placed above sand on a table, it glides over easily, offering no resistance at all.

Also on a road spilled with sand, bikers or cyclists have a high chance of slipping and falling.