tip recess?

There are probably other reasons, but mine depend on what I am welding.

Welding the outside of a 90 degree corner, I would extend the nozzle to almost touch the material, while maintaining a wider gap to the contact tube.

Welding the inside of a 90 degree corner, I would retract the nozzle to extend the contact tube closer to the weld.

Welding a flat plate, I would set the contact tube about flush with the nozzle.

It's all about keeping the shielding as close to the weld as possible, while maintaining about the same (proper) distance between the weld arc and the end of the contact tube.

It's called "stickout". As eloquently described by Maxwell, there are joint configurations where you want it to stick out or stick in.

Too much stickout can cause air infiltration and resultant porosity. Or you just lose that nice spray across the arc. Too short, and you overheat the tip, or the cup drags.

It is difficult, though, to have one cup fits all philosophy, though, as it doesn't. But unless one is welding only one type of joint all the time, it is ineffective to be changing tips every few minutes or sooner.

Tis best to find a tip that works under most conditions, or a position that allow for workable stickout.

Steve

There are considerations of joint design, but the main consideration is heat. When welding at low amps on thin metal (short circuit) you want to keep the wire stick-out short so you want the tip a little proud of the cup. For higher amperage welding (as in Spray mode) you want the tip flush with the tip and for high amperage spray welding you want the tip recessed or you will burn it off.