The purpose of hydrolysis of macromolecules is mostly the breakdown of polymers into monomers, we know that this happens constantly in our bodies, that makes the absorption of nutrients easier. The main difference between dehydration synthesis and hydrolysis is the fact that we have the building of molecules up in the dehydration systhesis process, and as we already know, the hydrolysis do exactly the opposite, breaking molecules down. Another thing to have in mind is that if we are building something up, we need energy to do that, so in dehydration it is required to have energy in the process, meanwhile in the hydrolysis energy is released.