Sharpen exaggerates the boundary between colors in bitmaps. It is useful for bringing out detail. It is not useful for:

Gentle transitions between shades of the same color.

'Noisy' bitmaps. Applying sharpening to such a bitmap is likely to emphasize the noise.

The Special Effects dialog box contains three levels of Sharpening - Low, Medium and High. One of these can be selected, and the slider used to set the level more accurately. For more details, see Applying special effects to bitmaps.