Once upon a time, many years after we moved to the house, the County decided that everyone had to have a gizmo at the water connection that would not allow "back flow" from their property back to the water supply. That gizmo, forced us to have that tank at the top added, so that we don't get push back from the County's gizmo. At least, that's what my memory tells me. If I ask DH, I'm sure he would know all the technical stuff...but, frankly, I'm not in the mood to listen.

If you need more explanation, I'll ask, another day & report back.
P.S. DH said, think of it as a shock absorber.