Wider spaced knobs are better for mud and wet sand or, for that matter, even snow. That is correct. With closely spaced knobs, the tires quickly clog up with mud and wet sand and lose their "bite". MTB tires tend to be very specialized in tread design and need to be chosen, carefully, for the types of trails you ride. The wrong kind of tread for your trail can greatly reduce performance and actually be dangerous. For instance, those big tall aggressive knobs may be just the thing for riding dirt, because the tire sinks into the dirt and the knobs can do their thing and bite. Not a good choice for riding on rock, though, or even hardpack, because then you're riding on top of the knobs and you actually lose tire surface area in contact with the trail and that can greatly reduce traction.

To make things even more complicated, it's also very much a matter of the tire compound used in the tire. Those old heavy duty nylon MTB tires we used to use were almost impossible to wear out, but they had so little give to them that they were harsh to ride and were notorious for lacking traction at times. Believe me, it's not just tread design, but also the compounds used. My riding on icy hard pack snow really shows the importance of this. Even with the same basic tread design on my different fat bikes, some tires offer much better traction than others on the slick stuff, the difference being that some tire compounds are stickier than others.

My trails are all dirt, now that I no longer allow myself to climb boulders. What works best for me are XC MTB race tires as long as the trail is dry. Plenty of traction, but, more importantly for my single speeds, great rolling speed. They don't call them XC race tires for nothing. Huge difference between riding these and an aggressive dirt tire. When things go wet and mushy, I just switch to one of the fat bikes and wait till the trail dries before riding the other bikes. These same XC race tires also do a great job on pavement and are ideal for gravel.

My 700c road bike single speed is my Nature Boy and it rides on 700x38 Gravel Kings. I lose some rolling speed, but not too much. Great tire, though, for wet pavement. Lots of traction, there.