Flattening an image merges all layers into one. This can be useful if you want layers to interact. For instance, if you've got something in one layer that you want to smudge into something in another layer, you can flatten the image and smudge the two together. It's also useful if you want to produce a final image out of lots of constituent layers to save complexity. I know that I can create images out of a good 10-15 layers sometimes, which can be very confusing. Every now and again, I'll flatten it down to make it simpler. As such, I can have a background image made up of 4 or 5 layers, but it is flattened into one layer called 'bg'. As another answer states, once flattened, you can't unflatten, so I usually keep a backup in case I want to edit the original.
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Flattening an image merges all layers into one. This can be useful if you want layers to interact. For instance, if you've got something in one layer that you want to smudge into something in another layer, you can flatten the image and smudge the two together. It's also useful if you want to produce a final image out of lots of constituent layers to save complexity. I know that I can create images out of a good 10-15 layers sometimes, which can be very confusing. Every now and again, I'll flatten it down to make it simpler. As such, I can have a background image made up of 4 or 5 layers, but it is flattened into one layer called 'bg'. As another answer states, once flattened, you can't unflatten, so I usually keep a backup in case I want to edit the original.
I hope this helps.
Flattening means combining all current layers into one concrete photo. With this, you can start new layers and edit the photo again.
PS You cannot un-flatten once saved