Agreeing to share revenue, increase their bargaining capacity for networks, sponsors, that sort of thing. Also helps smooth out the variance of performance of the collective, e.g. all the years the ACC was thought of as being bad due to no national contender, the PAC had Southern Cal, later becoming Oregon and Stanford briefly. When those 3 were falling off, Florida State rose briefly then Clemson. A conference made of both the ACC and PAC wouldn't have had the lack-of-contender perception problem over those combined years.