It works because a question is a stronger opening than an explanation. Dropping an audience into a scene already in motion creates an immediate gap — who, why, what just happened — and the mild disorientation is not a cost, it is the mechanism. People will tolerate not understanding for far longer than they will tolerate not caring.
The technique has a debt attached. You have to pay the context back, and payments that arrive too late feel withheld while payments that arrive too early waste the opening. The usual rhythm is to resolve who and where quickly, and to hold why until it can do structural work.
It is now close to mandatory in short-form video, where the first second decides everything. A piece that opens on a person introducing themselves has spent its most valuable moment on the least interesting information it contains.
What does in medias res mean?
Beginning in the middle of the action, with context supplied afterwards. The audience is oriented by consequences rather than by setup, which creates an immediate question.
Is starting in the middle always better?
Not always, but it is nearly always better than opening on introductions. The technique creates a debt — the context has to be paid back — so it fails when the answer arrives too late to matter or too early to have earned anything.