Its more than just doing other aircraft on the freq a favor.

ATC is required to maintain minimum distances between aircraft in different phases of flight for wake turbulence avoidance.

The best place to reference the info would be the JO7110.65. The controllers bible.

4 miles sep. behind a large
5 miles sep. behind 757 (757-200 has its own distance minima)
6 miles sep. behind heavy

As you can see, and as UAL briefly pointed out, the 752 is a unique case where it does not meet heavy weight minima, but due to its very strong wake turbulence generation, an additional mile (for 5 total miles) is added to the distance of aircraft in trail behind a 752.

(I had two final exams today, one in tower ops, and one in tracon ops... its still fresh..... for the moment. )