In laymans terms rakeback can be considered exactly the same as cashback.
To a British person who is not a poker player it could be seen as the same as cashback on your credit card; collecting Tesco clubcard points, or Airmiles.
But while you may have to wait a year for the cashback on your Egg credit card purchase, or months to use your Tesco vouchers, rakeback is paid into your account on a frequent - often weekly - basis, and is immediately available to be used.
You wouldn't go to Tesco and NOT swipe your clubcard, so why play poker without rakeback? It makes no sense. It's stupid.
Only an idiot says 'no' to free money!