So that there is no way for a TSO to open it for inspection without asking you for the key or combination. This prevents surreptitious theft at the c/p.

TSOs at the c/p generally don't have those master keys, but with the elaborate theft rings that have been uncovered at several airports, it pays to be cautious, particularly if your carry-on contains your wallet and/or valuable electronics, from iPads to industrial tools, as so many FTers' carry-ons do.