Krak grenades against troops...

This is why they're free for new marine lists ;-)

But yeah, U-Marine Tyranic War Vets get to use them against big nids as a Str6 attack at 6+ to hit...because they;re too cheap to give the dudes some powerfists...but otherwise, they;re vehicles only.

Think of it as an adhesive shaped charge. If you could somehow manage to stick it onto a moving target who's trying to cut your head off, it could be effective, but throwing it won't do any good, and you're better off just fighting with your weapons than trying to wrestle someone down and stick a bomb to them.

Really, it's just a big colorful picture book. Page 4 has a picture of a Krak Grenade. In the bottom right corner there's a silhouette of a tank with a green check mark and a big yellow smiley face on it, and in the bottom left there's a silhouette of an ork with a red X and a yellow frowny face.

In the advanced manual they give to veterans it simply says "Place bomb on tank. Run the fuck away. Do not wrestle with the bomb. Do not eat the bomb. When in doubt as to what to do with it, use your fucking gun instead."