I can actually understand this - suppose I was an agent and I made up a random name, like 'Polly-O string cheese'. If I used it consistently, a spy for the other side could do traffic analysis - things like " 'Polly-O string cheese' always gets a coffee, except for 2 recent periods of about a week each. Suspected agent X was reported as being in country Y, an ally of ours, during those 2 periods, and at no other time. Next time 'Polly-O string cheese' doesn't get a coffee, if X is in country Y, get the Y state security to arrest him.
If I were agent X, I would be very nervous at having to give any name, even if I could make one up each time. Humans are not very good at making up random things...