The aspect ratio mismatch everyone runs into
Footage shot in landscape (16:9) doesn't fit a vertical platform's 9:16 format, and footage shot vertically looks awkward with huge black bars on a landscape screen. Rather than reshooting, cropping lets you cut down to the portion of the frame that actually works for the target aspect ratio — sacrificing the edges of the frame to gain the correct proportions.
This is the single most common reason people crop video: content shot for one platform needs to be reformatted for another, and cropping is almost always faster and cheaper than reshooting from scratch.
What you give up when you crop
Cropping necessarily removes whatever's outside the new frame boundaries — there's no getting that content back once you've cropped and exported, so it's worth being deliberate about what stays in frame rather than cropping reflexively. If the subject you care about isn't centered in the original footage, cropping to a narrower aspect ratio can accidentally cut them out entirely if you don't check the framing carefully.
For footage with important content near the edges — text overlays, a second person in frame, background context that matters — cropping to a significantly different aspect ratio may require choosing which part matters more, since you genuinely can't keep everything.
Cropping vs. resizing: a common confusion
Cropping cuts away parts of the frame to change its shape, while resizing scales the entire frame up or down without removing any content — the two are frequently confused but solve different problems. If your footage has the right aspect ratio already and just needs to be a different resolution (say, 4K down to 1080p), that's resizing, not cropping.
If the aspect ratio itself is wrong for the destination — landscape footage needing to become square or vertical — cropping is the right tool, since resizing alone can't change proportions without either stretching (distorting) the image or adding black bars.
Cropping and re-encoding go together
Unlike a simple rotation, cropping requires re-processing every frame since the actual pixel content is changing — this means cropping always involves a genuine re-encode, with the usual small quality trade-off and processing time proportional to the video's length and resolution. It's not an operation that can skip re-encoding the way some rotations or trims sometimes can.