Less recoil so you can spot your hits/misses. Barrel life is about the same and 6mm bullets are a tad bit cheaper than 6.5 bullets and a lot cheaper than 308 bullets.
Heavy bullets are ok, but I want light bullets with a high bc.
For a prone only match on paper targets like an F-Class (600-1,000 yards) match, I am using 215-230 gn bullets in my 308.