That's really not what it is or why they do it. It's almost always been a means of cultural infiltration either coinciding with or in advance of colonial invasion and occupation. Missionaries played an important role in enforcing colonial control over colonized people by indoctrinating them in European religious beliefs and European notions of their inferiority.

It also serves an important cultural role for the sect doing the proselytizing as it serves as a form of rite of passage for youth, and a way for the community to re-affirm and justify it's own beliefs by doing "Good works" or "Saving souls" or however they conceptualize it. Most Western missionaries in the modern day are poverty tourists with religious delusions and will return to their community after a few months or a few years at most.