Reason is that competitors might trigger false emails to our support just to check how many cases we're getting, say, in a week. Monday = Case # assigned is 354, Friday = Case number assigned is 454, that means 100 in a week.
One of the softwares I've tested (whose use I dropped for FogBugz) has this mask, so that I can define my case number as, say, yyyymmddcccc, where cccc is a random number.