A beat sheet is not a summary. Each line states the turn: what a character wants going into the beat, what stops them, and the state they are in coming out of it. A list of events that does not name what changed is a running order, and a running order cannot be diagnosed because nothing in it can be wrong.
The reason to build one before writing is economic. A beat is a sentence and costs nothing to throw away; a draft costs a day and you will find yourself defending it. Every writer who has argued for keeping a scene they knew was dead was arguing for the hours he had already spent on it.
In generative production the beat sheet does a second job: it is the document the shot list is derived from, so it decides how many shots exist and therefore what the piece costs. Changing a beat is free. Changing it after four hundred renders is not.
What is the difference between a beat sheet and a synopsis?
A synopsis says what happens. A beat sheet says what changes at each step. The want, the obstacle, and the state at the end of the beat. Only the second can be tested for structural problems.
How many beats should a short film have?
A ninety-second piece usually supports six to nine real beats, and most drafts carry twice that. Past the ceiling the beats have become events, which is what makes a short feel busy and empty at once.