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I remember when layers first appeared in page layout programs; they really changed the way we worked. Layers allow you to put similar items together on a layer that can be selected, hidden, locked, or otherwise manipulated.
What’s sad, though, is that people nowadays don’t take the time to use layers. They work with all their elements on a single layer, and then struggle to select and separate groups of objects.
If you have a document that has a lot of images on each page, you may want to hide those images to help you work faster as you scroll around the document. That’s when it makes sense to create a layer that contains all the images. With all the objects on the same layer, you only need to hide the layer itself.