Stick / tig are Constant Current processes. You set your amperage and voltage varies based on arc length.
These concepts are a bit advanced, but allow the welder to have more control over the weld process. In mig, especially with machines with fixed taps ( dial you turn from say 1 to 6) you can get "in between" settings by adjusting how far away you hold the gun while welding. This can be helpful when say welding thin sheet metal and setting 3 is too hot, but setting 2 is too cold. If your usual stick out is say 3/8" from the work, moving in to 1/4" will make the weld a bit "hotter" so maybe you can get Tap 2 to work. If you increase the stick out to say 1/2" the machine will weld "colder" so that might work on say tap 3. Another place this can be helpful is where your joint isn't even. Say the gap closes down to nothing or opens up to 3/16" wide. If you push in and decrease the stick out when the gap tightens up, it will act like you turned up the machine slightly and allow you to burn in better. If the gap opens up, backing off some will cool the puddle and help you fill better without blowing holes.
All of this 1st assumes the welder has good control of all the variables. I often see new students who don't realize that they are changing their stick out as they weld. You see it most either when welding round objects, or when welding vertical. With vertical, many times students want to hold their hand "still" and roll their wrist as they move up the joint. That means the stick out is long as they point the gun down, then it shortens up as they get the gun close to parallel with the material, then the stick out lengthens again as they continue to rotate upwards.
What you want to do is set your hands so that the gun angle ( and stick out) remains constant, and move your hands up consistently. It is possible to roll your hands and get good welds, but if that is the case, you have to move the gun in and out as you do so to maintain the same consistent stick out. You also have to make other adjustments to deal with the gun angle changing, but stick out is usually the biggest issue when this happens.
This combined with other "tricks" like varying travel speed to heat/cool the puddle, pushing vs pulling when doing solid wire ( we teach all our students to push vs pull as it's easier to later get them to drag vs having to "reteach" them to push) all are tools in the welders tool box and are often what separates those who can weld from those who just squirt metal in the general direction of the joint.