These are all hugely underrated uses of mobile phones. Having drawings and documentation to hand, downloading manuals for random pieces of equipment without leaving the field, being able to stick your phone in behind equipment to get photos of stuff you can't see, talking to network equipment... This stuff can save days of wasted messing around.

Also just quickly recording things. Terminal block wiring inside the case of a VSD or other equipment (so when later you think 'oh wait, what was wired in to X5' you don't have to isolate the damn panel again to get in and check). Mark up drawings and take a photo to document it. Take a photo of that scribbled-over page of the manual that the client's been using for 10 years to configure their product and that's not recorded anywhere.