Based on the composition of phloem tissue, it is most likely used by the plant for

The phloem is a vascular tissue or a transporting system used by the plant for it to be able to undergo functional photosynthetic functions to help it produce its own food and energy.

In layman's terms, the phloem is the vascular system of the plant (like our blood vessels) that is responsible for transporting water and other nutrients (i.e. sugar, proteins, mRNA) which come from the soil to be equally distributed to the whole plant.