From what I see most people even students don't understand proper distance for most techniques. This is mostly for striking arts. Distance in the grappling arts are a bit different and even more complicated, it is not just being in touching range.
Because I box and do TKD and my grappling is limited I will defer to Steve BJJ for distance when it comes to grappling.
Most strikers tend to get or stand to close for their techniques. This is why you see knees into thighs a lot. You will also see knees colliding with knees and knees hitting feet. Many times you see kicks that miss by going past the target as well. Also you will have punches that do no damage as the punch never got extended. I don't know how many times I see a punch that pushes the opponent back. Because the arm was bent at impact by 90 degrees or more, then they extent the arms out pushing not punching the opponent. That is why untrained people can throw 10 or more punches standing toe to toe and miss all of them.
Distance if very hard to teach because people tend to want to be in grabbing distance not striking distance. They also tend to think that the front hand and foot is the closest to the target when that is not really the case. The rear foot and hand are the closest to the target. But it is almost impossible to get people to understand this. Don't believe me? The do the following.
Stand in a classic fighting stance. That is one foot slightly in front of the other on a 45 degree angle. I will start with the feet first as this is simplest to explain. Now from this stance just lift the front foot and and kick in front of you as far as you can without moving your back foot forward. You can pivot the back foot to get as much distance as you can but do not move it forward. Note how far you kicked out.
Now in the same spot in the same stance simple pickup your back foot/leg and kick towards the same target. Do not move your front foot forward but you can pivot on it to reach out as far as you can. Note the how far out you kicked.
If you did this correctly you should have kicked out the longest with your back leg. This is because the foot or leg that you are standing on when you kick is actually your point of origin. So pickup your front leg you lose about 2 feet of distance. Pickup the back leg and kick and you gain about 2 feet of distance. Thats 4 feet in all. 4 feet is rather long but not many kick from that distance without taking a step. The great fighters understand this and start kicks from a range that many think they are safe but are not. That is why they seem so fast. Boy he kicked me from way back their, he must be fast. No, he has timing and distance on his side.
Now for the punch. It is similar but a little different. Again stand in your classic one side forward fighting stance. Now jab out as far as you can with the front hand. Note the distance you reached. In that same spot and same stance punch out with the back hand. When doing both punches turn your hips as much as you can for maximum distance. Again if done correctly you will have almost 6 inches to a foot of greater distance with the rear hand. This again is due to the point of origin of your feet. Because your front foot is closer to the target you greater range punch will be with the rear hand not the front hand.
The concept above in not understood by many as if you put 3 feet between your front foot to your opponent front foot it seems to far to strike but it is not.
I hope this helps because this is the one single thing I try to get across to all my students is distance. When the target is moving away from you don't use your front leg or hand to try and hit it, use your back leg.
This is just one concept pertaining to distance, there are so many more. But understand this one first and the other seem to fall into place.
Forgive any bad typing as I typed this as my brain was thinking it and I know there will be many grammar errors as we tend to think faster than we type. I am to tired to go back and proof read it also.