So, how exactly does an online cashback service make us of blockchain and cryptocurrency?? 
The blockchain is used to store all of the data on transactions and other things. It's decentralized, so it's hacking-proof. I think the crypto is there just cause it's popular
I myself don't really see the nee for crypto in such projects.doesn't the use of cryptos offer any benefits or security or something?
Kind of. It makes you independent of banks with their transaction fees and slow transaction speed.
Makes purchases anonymous too, if you're into that sort of thing. Tokens can grow in price, thus you can earn money by buying them early and selling late. Off the top of my head.
Bit clearer now thanks. I'm kind of new to this technology.