In the past it was noise abatement.
I haven't flown for many years but in my day it was not uncommon to receive a vector that would take you to the OM along with a clearance - cleared for the approach. The last assigned altitude was well above the glide slope intercept altitude at the OM.
The controllers procedures did not allow descent below that last assigned altitude due to noise abatement in the area of the vector. Thus the pilots had to start a descent before intercepting the approach path in order to make a normal descent to the airport. This was most prevalent in NY, Washington and other high density areas.
It was still common to receive these types of clearances when I retired in 1998.