One gets 'improved performance' in areas (like looking at the sky) where the game otherwise would go above 30, 40 or 60fps etc. Thus increasing the average fps ofcourse. Once games will become like UE5, thus straining the systems more consistently, then its a good feature to smooth out those dips below 30fps, for example.
Question now is, do Digital Foundry and other youtube channels start benchmarking with uncapped framerates in the future? I'd be all in for that. Average fps's skyrocket in special on powerfull pc gpu's, doom eternal for example can reach in the 500/600fps range quite often, increasing the avarage fps dramatically.