Assert is a way to make a tracked or blocked value act as a move instruction on playback. It is often used in a multiple cue list environments, or to assure that a transition happens in the desired time. See Using Assert on asserts in multiple cue lists and Assert (Fader Control) on using assert for playback.
Assert can be useful in a single cue list.
Example:
Cue 10 is a blackout with a zero count. It is common practice to block blackout cues. Let’s say though that some of the lights were fading to zero in cue 9. You hit [Go] for cue 10 before cue 9 is finished. The lights that were fading to zero in cue 9 will continue their downfade in the timing for cue 9 since cue 10 doesn’t provide them with a new move instruction. If you assert cue 10, the lights will bump to black as expected.