Reason behind geometry of heater blocks?
A heater block has its shape designed simply by the necessity of the needs and the ease to manufacture them. Functionally, a heater block serves as:


a structural component holding the following components in a well-known position to one another:


heatbreak
nozzle
heater cartridge
thermosensor

a transmission medium of heat energy from the heater cartridge to the thermosensor and filament path.


this dictates the use of high thermal conductive metals  

a thermal energy storage to equalize the heating pattern of the heater cartridge.


While this dictates the internal geometry of the block, it does not say anything about the external geometry. This is chosen entirely because of manufacturing, which strongly prefers square items in a vise with stop positions to allow repeatable machining.

While other heater designs are existant, for example wrapping a heater wire around the meltzone, the use of a machined structural element that houses bought components saves a lot of time in assembly Quality Control and subsequent repairs, as each component (heater, thermosensor, block) can be rejected or replaced on its own. This makes the machine all in all more serviceable.