Sometimes its not possible/feasable to turn the power off.
The main reason I tape outlets/switches is so I know that I did the job/was the last one in there if I have to come back 5 years later to look at something,it also slows idiots down from messing with things.
Its a habit from left over from my early days in the trades doing commercial work with metal boxes and plaster ears I guess.
And yes I do tape wire nuts on in machinery that rattles/shakes a lot,Again it also keeps maintenance monkeys from messing with stuff.