If you don’t use a scale every time you add or remove refrigerant I would suggest you begin doing so immediately if not sooner. Weighing in while charging is fairly obvious and is useful so you can keep track of what you are using and how much to charge a customer.
When you have a system that has just been repaired it is a good practice to weigh in the charge to factory specs plus or minus adjustments for the line-set if it is a split system. This is all pretty evident, but why would you weigh a charge out? There are many reasons but one good example is whenever you have a failed compressor, weighing out the charge can help indicate whether possible undercharge or overcharge may have contributed to the failure. With any significant failure on an older system, weighing out the refrigerant can indicate whether a leak is likely. When possible on major failures you could even weigh out the refrigerant at the time of diagnosis just to ensure that a leak or a compensatory overcharge may be at play.
Using refrigerant recovery as a means to find possible cause or even diagnose leaks on non-functional systems is next level diagnosis in my book. Use your scale.
Weigh in when adding charge
Weigh out as a diagnostic aid and to ensure you don’t overfill your tank.