What purpose is served by the loss of an - H and - OH end from two molecules as they join together during dehydration synthesis

The loss of the hydrogen on one molecule produces a negative charge, which is attracted to the positive charge formed by the loss of the hydroxy group from the other molecule.

To create positive and negative ends to attract and hence combine with each other.

Explanation:

Two molecules combine usually by some sort of attraction between them.

When one molecule loses OH - ion, it becomes positively charges while the other molecule loses H + ion and becomes negatively charged.

Now two molecules, one electro positive and other electro negative attracts each other and combine by forming hydrogen bonds.

Like wise the H + and OH - released, also combines to form water.