The main benefit of ROP ads is they can cost quite a bit less than other print ads. (mediaspace solutions)

But Gill Randall, joint CEO of Spark Media Group, explains that on a national level a massive shift has been seen from Run of Press (ROP) to inserts due to the volume game. (themediaonline)

Magazines usually sell three primary types of advertising: run of press (ROP), mail order, and insert. Most magazine advertising pages and revenues are derived from ROP ads, which are printed within the magazine. (Industry Surveys Publishing & Adv.)