Kinda useless today. Systems are so small an ac machine is the only way to go.
If you want to service an A/C properly you will need a way to remove and store the refrigerant, evacuate the system, check for leaks and then fill by weight.
Now if you have a slow leak in an older car, you can dump a can of R134a every once and awhile. If you stay away from stop-leak you should be OK.