I've always wondered. . . after all ER has ladies-in-waiting to deal with carrying things. Does the Queen have credit cards? A driver's license? Pictures of the corgis?
I think she carries a handbag so that when things are particularly trying, she can open it and take a quick, reinvigorating peek at this little button which she has affixed to the lining.
A deep breath, a stiffened upper lip, and the House of Windsor carries on.