The gap between recording and the moment that matters
Almost every raw video clip has a bit of waste built in — the second before you actually started talking, the fumble reaching for the stop button, a few extra seconds of nothing at either end. It's rarely worth opening a full editing application just to shave those off, but leaving it in makes the clip look unpolished and wastes a few seconds of everyone's attention.
Trimming solves exactly this narrow problem: cutting from the start and/or end of a clip without touching anything in between, which is a much simpler operation than full editing and doesn't require re-thinking the content itself.
Trimming vs. cutting the middle out
It's worth being precise about what trimming actually means: removing time from the two ends of a clip, not deleting a section from the middle. If you need to cut something out of the middle and stitch the remaining pieces together, that's a different operation (splitting and rejoining) that needs more than a trim tool can offer on its own.
For the common case — a clip that's basically right but has dead time at the start or end — trimming is the fastest and simplest fix, and it's the right tool for a large share of everyday video cleanup.
Why trimming can be fast even on long clips
Depending on where the cut points fall relative to the video's keyframes, trimming can sometimes be done without fully re-encoding the footage, which makes it dramatically faster than a compression or format-change operation on the same file. When a re-encode is required — often because the cut point doesn't land on a keyframe boundary — processing time scales with the clip's length and resolution as usual.
Either way, trimming is one of the lighter video operations, and even a several-minute clip typically finishes in a reasonable time compared to operations that touch every frame, like compression or format conversion.
A quick habit that saves re-work later
If you're planning to trim and then do something else to the clip — compress it, convert its format, add a watermark — trimming first means every later step processes a shorter file, which speeds up everything downstream. It's a small ordering choice that adds up when you're running a clip through multiple tools.
Keep your original untrimmed file if there's any chance you'll want a different cut point later — trimming, like most video operations, discards the removed footage permanently once you download the result.