Why should the cutting edge of a knife be as sharp as possible?

Pressure is given by:

$\mathrm{Pressure}=\frac{\mathrm{Force}}{\mathrm{area}}$

The pressure is increased when the area on which the force is exerted is decreased.
Therefore, the cutting edge of the knife should be as sharp as possible to reduce the area such that greater pressure is exerted on the object and it is cut easily.