Different tyres suit different conditions, hard terrain tyres require more knobs with a larger, shorter block close together to increase the footprint which gives you grip. Soft terrain requires fewer knobs with a smaller, taller block which are spaced further apart to allow the tyre to dig into the soft terrain giving you more grip.

Similar to muddy conditions the added advantage of larger spacing between the knobs allows the tyre to self-clean easier. As the tyre spins the mud collected will fling off so the clean knobs bite into more terrain every revolution.