I always though that Riker was kept around post-Best of Both Worlds to keep an eye on Picard. But, in-universe, that only works for a fairly small period of time.
Not too small a period of time, as late as First Contact there's indications that Starfleet Admiralty did not
completely trust him. And Insurrection hinted that Picard was being kept out out of combat during the Dominion War.