Networks love it because it's an easy way to increase an audience, and while it may have been very popular in the 1980s and 90s, such as when Paul Reiser from Mad About You showed up on Seinfeld or ...
Many popular sitcoms of the era shared a world with a similar show on the same network. That's how we got Family Matters' Steve Urkel (Jaleel White) crossing over to show up on Full House, or Paul ...