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

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.