On occasion, a toilet's drainage stopper can have a bit of floating problem, in that it won't be totally airtight. This will make the stopper float upward, causing water to continuously drain out of the tank. Enter scrub brush to save the day (and the water bill...)!
The only issue with this "lock" is that it's less of a security mechanism and more of a test of strength between you and the person coming in. If a 300-pound linebacker gives the door just one good yank, he'll probably send you flying from the toilet seat with who-knows-what strewn all over the floor.