Nonsense and a rip off. Refrigerant does not degrade. It will work fine for ever unless a contaminant enters the system or the refrigerant leaks out. There is no "purge." What they'll probably do is maybe add some unneeded R134a to your system or maybe pull out your refrigerant charge into their recovery machine and be sure the correct amount of your same refrigerant is put back in. In either case, don't. If there is contamination in the system it may be purged with an inert gas like CO2 then drawn down to a deep vacuum before recharging. This is only in the case of contamination, not due to a weak output.
I have a reefer tech certificate. I've run and repaired systems using R-11, R-12, R-22, R-502, and R-134a. Centrifugal, piston, screw, and scroll compressors. Never, ever had degraded refrigerant. Any loss of capacity was due to something I had to repair. Once I found and fixed the problem, and put the same good old used refrigerant back into the unit, it ran like new.