When you see something every day, you no longer see it.
Think about it: you hang a framed photo of your family in the entryway, and since you pass it every day, it begins to blend into the wall. It becomes one with the wallpaper.
Familiarity breeds forgetfulness.
But take that photo off the wall, and look at it again, and you will see it in a completely different light: "Oh, little Kyle still had braces. Wow, glad I switched hairdressers since then. Ugh, look at that funky blouse; well, that was the style."
In removing the photo from its native environment, so to speak, you see it differently. And once you see it differently, you can determine if it has become clutter to you, or is something you want to keep.
Much of the clutter in our homes is hidden from us in plain sight, like that framed photograph. We pass it every day, sometimes many times a day, camouflaged by its long-standing presence. It has become so familiar that we forget it's sitting there on the wall, or the shelf, or in the corner of the room. It blends in with the surroundings.
Look around the room you're in right now, and find the clutter hiding in plain sight.