Question

why dried resins are kept in the water for a few hours before their use

Answer 1

so, that the raisins get swollen by the process of imbibition.

Answer 2

Dried raisins mean they do not have water content. Before use, raisins are put in water for sometimes so that they absorb water from their surrounding and swells.

The water moves inside the raisin by osmosis. The water moves from high concentration to lower concentration through the membrane of a raisin to balance it.