When you need to flip image files
Three cases account for almost all of it.
The first is selfies and video-call screenshots. Phone cameras generally show a mirrored preview because that is how you see yourself in a mirror, and some apps save the unmirrored frame. The result looks slightly wrong to the person in it, and a horizontal flip fixes that.
The second is composition. A subject looking or moving left reads differently from one looking right, and in a layout you often want them looking into the page rather than out of it. Flipping is the cheapest possible fix.
The third is scanned or reversed material: film negatives photographed from the wrong side, screenshots from a mirrored display, artwork that needs to face the other way in a spread.
Check this before you mirror image content
Text. A horizontal flip reverses every letter in the picture, and a vertical flip turns them upside down as well as mirroring them. This is not a limitation that can be engineered away, because the tool is moving pixels rather than understanding the image.
So look for text before you commit: signs, logos, watermarks, number plates, a brand name on clothing, UI captions in a screenshot. If any of it matters, flip is the wrong operation, and cropping or rotating is probably what you actually wanted.
Faces are the other thing worth a glance. Human faces are not symmetrical, and some people find a mirrored portrait of someone they know quite uncanny even though they cannot say why.
Flip photo horizontally, vertically, or both
Horizontal is the one people mean nine times out of ten: left and right swap, which is what a mirror does.
Vertical flips top and bottom, which for a normal photograph produces an upside-down image with the light coming from the wrong direction. It is genuinely useful for reflections, for a symmetrical pattern, and for material that was scanned the wrong way up.
Both at once is the same result as a 180 degree rotation. It is offered because the two operations arrive at the same place, and whichever you were thinking of is right here.
Format and quality
Each file keeps its own format, so a PNG stays a PNG and a JPEG stays a JPEG. The exception is a source the canvas cannot write, such as AVIF, GIF or SVG, which comes out as PNG.
Everything runs in this tab. No file is sent anywhere, which also means a folder of several hundred images is limited only by your own machine.