This is a direct solution to the rockpile/fragmentation/shotgun problem. If the warhead is planted on the surface of the asteroid/comet or close above or buried in it, then there will be strong coupling of a schock wave into the body, and disruption and ejection of "Hot" debris is likely.

If there is some reasonable standoff, then a lot of the energy is wasted, but the surface is heated in a farly uniform way, and then boils, causing a weak rocket effect. Enough stand-off explodsions *might* generate enough orbital change to deflect the asteroid (This is a man-made version of the Yardowski effect that causes small asteroid and Comet orbits to change due to solar photon pressure!)

The fusing of the surface is a side-effect, not the principal purpose.