The "Redeem Rewards" function is a fairly recent change to Steemit. Previously rewards were paid directly into your account balances and this step did not need to be done. The change was made simply to help the blockchain run more efficiently, as it reduced the number of transfers/transactions happening since not every little .001 Steem was being sent, instead being accounted for in this ledger type balance.
Click it whenever you think about it, there's no harm in claiming often and in fact there is no benefit to just letting it sit there unclaimed. The sooner it is in your actual account balance the sooner it is earning interest for you or making your votes more powerful (if it is Steem Power.)