No need to sharpen: with thin enough lead, you don't need to sharpen your pencil, because the line will never be too thick.
Consistent line: especially important for engineering and technical drawings, if a mechanical pencil is held upright, the line width will stay exactly the same - the width of the lead. If you need a 0.5mm line, a mechanical pencil can provide it predictably, a wood-cased pencil can't.
Balance: many artists use mechanical pencils for this reason. With a wood-cased pencil, as it's used and sharpened, it gets shorter and shorter, and the balance of the pencil changes. Eventually, it becomes too short to use comfortably, and is thrown away.
Refillable: rather than being used up and disposed of, the pencil can have more leads poured into the top. The same pencil can usually be used for many years, depending on the quality.