One point I haven't noticed being mentioned here, is that taxying time is engine run time, and this affects the fuel reserves and overhaul period of the engine/s.
I hereby put up one classic case, which involved a twin and which resulted in multiple fatalities.
The twin was being fitted with specialised equipment as part of a new job. This installation was a one-day project. The pilot planned to make a flight on the day following the installation, so he fully-fuelled the aircraft.
The LAME fitted the equipment, but had to start and run the engines, and taxi the plane, as part of the new equipment installation testing. This took considerable time.
The LAME did not log the hours involved in engine running and taxying, as he wasn't endorsed to fly, and wasn't the PIC of the aircraft.
The pilot arrived the following morning, received instructions that the equipment was fully installed, and ready to go.
He did not check the fuel tanks, because he knew he had filled them the previous day - and there was no written record of any taxi time or engine run time.
The pilot commenced the flight, and within a relatively short period after takeoff, the plane crashed in heavy forest with the loss of all crew and pax, due to fuel starvation.
In the subsequent inquiry, it was revealed that the engines were run for a substantial period, around 3 or 4 hrs, and that this was not recorded anywhere.
It was deemed crucial in the resulting investigation summary, that all future engine run time, be recorded in the aircraft log book, regardless of whether the aircraft was airborne or not, and regardless of the endorsement/s of the person running the engines.
Yes, the PIC was negligent in not re-checking the tanks, immediately prior to takeoff. However, this neglect, which directly caused the resultant crash, could quite likely have been negated, if the notation of engine run time had been placed in the logbook.