What is the purpose of the rear end of a roofing hammer?
One reason for the offset design for the type of roofing hammer that you showed it that it can be used to guide or tap the roof covering piece into place.



The type of hammer with the offset spike end can be used as a shingle tab exposure guide. The point slid under the shingle being applied with the offset step at the edge of the shingle the hammer head would be adjusted till it is even with the previous course of shingles. Some types of roofing hammers have an adaptation of this idea to allow for an adjustable shingle exposure.