It's anonymous, so marketers and other "interested" parties can't track your spending habits. It's useful for personal transactions, like shopping at garage sales or paying the neighbor's kid a few bucks to roto-till the garden. It is universally accepted. It doesn't put your accounts at risk. And you always know how much you have left to spend. There are no overdrafts - if you run out of cash, you stop shopping, so you can't put yourself in debt or overdraw your account.
I don't really see any downside to using cash.
If you're getting your cash from your bank, fine, but ATM fees of $2.50 on each side of a withdrawal mean you're paying 2.5-5% for the privilege of taking $100 of your money out of the bank.
Not a bad deal for the banks to skim every withdrawal.