From what i understand deification was, on the most part, adopted as a means of propaganda by the rulers. It stemmed from Greece and Egypt.
Greeks kings such as the Seleukids said they were of divine ancestry and the Ptolemies and the Pharoes said that they were actually gods. Kleopatra and Caesar presented themselves as Isis and Osiris, with their son Caesarion as being the god of the universe , Horus. Thus the dynastic cult was completed.
Being seen as divine, or having divine favour, was a means of legitimising the rule of foreign or non-dynatic rulers. Also, was used as a means of unifying the empires.
Although, it is interesting to note that having divine honours did not automatically mean that they were thought of as deities the same way as Apollo or Zeus. Often, honours were given as a means of showing appreciation and respect to the rulers for benefits being granted to the cities or states.