In part, it helps them analyze how the various shows are doing- how well do they pull viewers independently and in concert. That helps them decide renew/cancel and show scheduling.
A big part of that would be considering what they’re curre scheduled against. Counter programmed against weak shows, their shows look stronger, and thus better to advertisers. Program against a heavy hitter, and your show could look so weak that the advertisers flee and kill the show.
Personally? in all my TV watching history, there have been precious few crossover events I enjoyed, and I haven’t bothered with any of CW’s. The Flash is the only one of the DC shows I’ve seen more than 2 episodes of, for a variety of reasons.
(None of which have anything to do with my perception of the shows’ quality.)