Most lenses provide sharpest results when used in the middle part of their aperture range. Lenses usually have performance problems when used wide-open. Stopping down helps a great deal, but once the aperture becomes too small then an optical phenomenon known as diffraction comes into play and the quality deteriorates once again. So most lenses work best at around f/8 or f/11 or so.

Naturally this sharpest point (some people call it the “sweet spot”) varies from lens to lens, so some testing would be required to find what’s actually best with a given lens. And the effect is generally more pronounced with less expensive lenses. Really high-quality lenses are often almost as sharp wide open as they are stopped down.