I've been training in martial arts since the 1980s and consistently since the '90s. I am a 2nd-degree black belt in Kenpo Karate.
In fighting, it is very important to judge range and understand range, because range determines both whether your opponent can pull off a technique on you and whether you can pull one off on him. To understand distance and timing is a fundamental principle in martial arts. Without it, you cannot fight.
Here we will only cover the three basic ranges of combat. These are long-range, mid-range, and close-range.
Long-range is also known as kicking range. In other words, you can kick someone from this range. It's a good range to fight from if you want to keep the opponent away or if you just aren't in reach to hit him with your hands. At this range, you can deliver your kicking techniques, utilizing whatever foot maneuvers are necessary to pull off your kicks.
Mid-range, or punching range, means you are close enough to the opponent to hit him with your hands. Of course, he's also close enough to punch you too. At this point, you are close enough to grab him, or at least potentially grab his arm. You can also use a shuffle to get into this range. Your opponent might even move himself into this range. It is important to note that the closer you are to your opponent, the more likely it will be that he can hit you. So, some measure of safe-guarding against this possibility is necessary; you must keep your guard up, grab him, or increase your own attack on him to check him and keep him from hitting you.
Close-range is the closest range where you both can hit each other and there is very little read time because everyone's weapons are so close. Here it is crucial to grab a hold of your opponent and take control, or else he will hit you to his heart's content, unless you are hitting him. Or you both might exchange blows. But the idea is that you don't want to get hit.
So it is important to understand how to use your feet, by moving them, to get in position to pull off techniques. Without being in the right position, in the right range, you will not be able to do what you need to do. It is also important to understand distance and time, not only in terms of your own timing and the "when" of when you can do a techniques, but you must also understand that you have less time to see attacks coming when you are closer to your opponent. In other words, distance determines timing. It will take longer for your attack, or his attack, to happen the farther away you are from each other. The closer you are to each other, the quicker that action will happen, so more contact and action is necessary. You must hang on to the other guy or hit him relentlessly. You don't want to just stand there and do nothing at close-range, or even at mid-range, because, as has been stated, the other guy is close enough to hit you quickly.
So to be a successful fighter, you must understand ranges, which means understanding distance and how timing relates to it, so that you can pull off techniques in the heat of the moment.