Main function of lenticel is : -

In woody stems, the epidermis is replaced by cork (periderm), which is impermeable to gases and water.
At certain places, the cork cambium produces loosely arranged cells (complementary cells) instead of compact cork, forming lens-shaped openings called lenticels.
Lenticels allow the exchange of gases ($O_2$ and $CO_2$) between the outside air and the internal living tissues of the stem. Some water is lost through them (lenticular transpiration), but this is not their main function.
So the main function of lenticels is gaseous exchange.