It means losing a lot of stuff, yes. It sometimes helps because some things slow down as state is accumulated. So resetting can speed things up, at the cost of

It means losing a lot of stuff, yes. It sometimes helps because some things slow down as state is accumulated. So resetting can speed things up, at the cost of losing all of your state. But the comparison point is switching browsers, which also loses lots of state. (In both cases, you can import a subset of your state into your new profile, and you'd probably get most of the performance advantages.) I believe all browsers have been improving in their resistance to the accumulated state problem, but I also believe that all browsers are still susceptible to it.