I do not know for sure why the "Claim Rewards" was implemented. As I didn't see an explanation if there was one. One benefit is that it reduces the number of transactions. If you claim several rewards at once it only creates one transaction in your wallet. I don't have any association with SteemIt, Inc. So, I can't represent "why" they do things. :)

I do not think they go away, certainly not on any short-term basis.