I don't claim to be the authority here, but I think most people do it either because tehy saw it done before and assume it is correect, or tehy are a fraid theier trees are "too tall" and want to make them smaller before they break. Of course, we all know that's a load of garbage, but the ideas are still out there.
Which means there are people who want this "service", which is why some companies do it. The longer you do this job, the more times you will hear "If I don't do it, someone else will." Not a compelling argument IMO, but there it is.
And I believe the previous response was correct as far as the origin. Pollarding creates a continuous supply of branches that can be reached and handled easily, so repeated harvests of the two-year sprouts keeps one in firewood. Obviously, not many people are doing it for that reason now--they just have misguided ideas about what makes a tree strong and healthy.
keith