Locks the track so that you can't inadvertently drag it, or whatever. Also should lock associated plugin parameter so you can't accidentally change a setting with the mouse wheel when you think you are somewhere else. I find myself doing both all the time....
That's not quite my understanding of track freeze - the reason the track and controls are locked is that the audio on that track has been rendered so that all the PC has to do is play back an audio track rather than process a bunch of CPU-hogging plugins in real time.
Also useful for soft-synths... wait, you can't use those in Vegas.