In addition to Brians reply ......
With a gauge, you can readily 'return' to the same numbers after slacking, repair work, etc. The best is to dynamically tune, then take the numbers and use these numbers for future reference to monitor the system. Any 'ductile' changes that occurs in the system later on will show on the numbers.I find a gauge will get me to a good static tune faster than by 'eyeball', but such adjustments are only 'initial' that need to be 'tweaked' by actually sailing ans watching the mast, etc., Then I record such numbers when Im satisfied and use them as a reference for future work/tuning.