Not having any cash at all, puts one at the mercy of the banks computer and security systems, best to have something to fall back on when their systems are in trouble or are hacked.
As an aside, in "The Handmaids Tale, their society was cashless. men had an "M" on their cards and women had an "F" on theirs. When the government wanted to take power away from women, they made the cards with an "F" unworkable and transferred all the women's money to their nearest male relative. A corrupt government could do something similar in some countries.