Depth of field exists because a lens can form a perfectly sharp image of only one subject distance at a time: a single plane of focus.

For a point on that plane, light rays passing through the lens converge to one point on the sensor, so it appears sharp. For points closer or farther away, those rays do not meet exactly on the sensor plane. Instead, they spread into a small blur spot rather than a point.

That blur spot is often described as a circle of confusion. As long as it stays small enough, our eyes still judge it as “acceptably sharp.” The range of distances in front of and behind the focus plane where the blur remains acceptably small is the depth of field.

Aperture matters because a larger opening lets in rays from a wider range of angles, making out-of-focus blur circles larger. A smaller aperture narrows the ray bundle, making those blur circles smaller and increasing apparent depth of field.

So the reason depth of field occurs is: only one distance is perfectly focused, and everything else is rendered as blur circles whose size may or may not be small enough to look sharp.