When starting out, every one uses duct tape, its cheap and readily available. I remember years ago, going through miles of the stuff… However, its not without peril. Duct tape, over time, or after after 1000+ folks have trampled on it, or PAR cans have boiled it, of any such combo makes for a nasty mess of adhesive residue.
Its nothing that some terry cloth and Bug and Tar Remover cant deal with, short of PAR can heating… but it is an extra hassle. Its also a good thing to sched periodic cleaning of cables and stage gear if you do use duct tape… or eventually, the folks will be having birds when their hands get covered with goo during set up and tear down.
Also, some versions of duct tape can be pretty reflective which may or may not be annoying. Lastly, some brands tear sideways, backwords, and upside down… pretty much anything but straight.
Gaffer tape, otoh is super cool… its a lot easier to tear straight, has better strength & temperature specs, is almost always mat (not reflective), and doesnt leave a residue mess (well, dont leave it on forever LOL) However, its substantially more costly. Permacell 665 is the standard, albeit corporate changes have tweeked the name and such over the years, but it is the “right stuff”.
Some less common uses include…
Transit case marking, rather than stencils, ie if you move, or are a sideman, you dont want stenciled labels, you want something that can be changed somewhat at ease. Granted, if you are in a big enough league to afford case wraps, thats even better, but for most, gaffers tape works wonders.
Shoe traction can be improved, especially if wardroad consultants come up with a cool image they went to present. Far too often said image is not practical for choreography, as the shoes are for show, not for movement and as a result may be like walking on ice. Gaffer tape, or duct tape can work amazingly well in this regard.
First aid… but not right over the injury, but as a durable cover over traditional first aid bandages and the like. (sure… most just grab a towel and duct tape, but its safer to use sterile first aid approaches, and then duct tape).