That is to play OpFor (Opposing Forces (ENEMY)).
During Vietnam war, USAF, USMC, and USN pilots flew against Migs that had significantly different aerodynamic flight characteristics. The US fighter aircraft turn performance (turn rate & radius) was not as "good" as the Migs.
Fighter training in Air to Air Combat was against "similar" aircraft (F-4s flew against F-4s from the same squadrons).
Flying Air to Air training missions did not prepare the US pilots for the missions flown against the Migs.
To provide dissimilar Air to Air training, the Navy Top Gun instructors flew different aircraft, including, over the years, A-4M, F-5, F-18 (Against F-14s) and F-16.