Dont need to withdraw them. They are already in your wallet, just in its own eUTXO. Whenever you need it to pay for a transaction, or send it anywhere the eUTXO will be used/merged.

Say you have a 100 USD note and a 20 USD note. The 20 USD note is all of your staking rewards within your wallet. If you leave it for long enough that 20 USD note will turn into 100 USD note.

But as of now, you have to pay something that costs 110 USD – the 100 USD note will be transferred, and your 20 USD rewards note will be broken up into 2 x 10 USD notes. Leaving you with only the 10 USD rewards note left in your wallet.

If your wallet have enough USD without having to break into the rewards note – it wont get used, but its still in your wallet.