To wonder the purpose of a Sparks card

[Buck3t]

I mean, if you know it's purpose or have in some way received a benefit, please let me know. Because I use it. But never are the things I usually buy from M&S are ever with that piddly 20%

I have a gazillion sparks points and have just chosen 20% off bread, salad bags, tomatoes & socks It's pretty worthless butI do get 20% off everything regularly too.
My Mum has M&S banking/ credit card and gets more free hot drinks vouchers than she knows what to do with & money off vouchers £10/20/30.