Much of what is done in martial arts training has to do with learning what can be done from which position. You end up in that position or put yourself there, you need to know what is practical and effective once you’re there in that position.
Whether it is grappling or striking, you train to learn position; and how to get into position, unless you just happen to get there because of spontaneous actions of you and your opponent. Either way, you need to know which lock you can apply, which strike or kick to which targets will work, what you have to do to keep from getting hit; you must understand and problem solve from this position and know what is reasonable and viable.
You learn how to move to get yourself in position. You learn how to put your opponent in a position that gives you the advantage.
Consider this situation in which you might need to close the gap between you and your opponent; you will need footwork even to launch a kick, but at the very least you need footwork to move in on the opponent.
Opponents are out of range
Glenn does a cross-over foot maneuver, his rear leg crossing his front leg
As Glenn finishes the foot maneuver by stepping in with his right foot, he is in range to deliver a punch.
Using a relatively simple maneuver, in this case what is called the cross-over in Kenpo, you can put yourself in striking range of the opponent.
In another scenario, you simply want to put yourself in a better position relative to the opponent where he has minimum use of his weapons and less access to your targets, while you have access to his targets and full use of your weapons. Consider the Kenpo technique called Circling Destruction.
Joe slips the left punch and parries it with his right hand.
This double parry flows right into a back-knuckle to the ribs.
Joe shuffles to the back centerline and strikes the face or neck and hooks the chin with the left to control the head and open the right side of the neck for a strike. Notice this movement involves backup mass and chambering as you move into position so you don’t waste motion, adding strikes as you move, taking care of multiple things at once. Sophistication.
Joe finishes the shuffle by sliding his left/rear foot to position him in a neutral bow stance behind James and delivers a right chop to the neck.
Joe claws through the eyes with his right, maintaining a check with his left. and…
…he scoop kicks the groin, checks with his left and chambers with his right, as the kick bends James over.
…this puts James in position for the inward/downward elbow to the kidney, which is delivered with marriage with gravity as Joe lands after the kick.
So, in this example you defend yourself and then maneuver to the rear of the opponent and control his head; the opponent does not have access to your targets, unless he spins; but you have your hands up or on him and you are controlling his head. From your position you have access to his targets, so you strike his neck, kick his groin and deliver an elbow strike to the kidneys.
Essentially footwork is necessary in any fighting art; for defense, slipping punches and kicks or even tackles and take-downs; for getting yourself into a decent position so that you are zoned away from his weapons but have full use of your own while having access to his targets or can otherwise take him down to the ground; and footwork helps to put you in range for your own assault on the opponent; and finally, it’s just a good idea to move and know how to move in a fight, because you increase your chances of not getting hit–a moving target is harder to hit than a stone statue. This is why it is sometimes warned not to get caught flat-footed. It’s also important to remember, in this regard, not to be caught back-pedaling or rocking back on your heels–that’s when the opponent will have an advantage and get you on the defense while he’s all offense.
All fighting arts make use of footwork; boxers, for instance, constantly use it to get in a position to hit the opponent or to slip and evade an attack. This is true of any realistic martial artist.