Because it leaks out. Period, no other reason to add, it doesn't get used up or worn out or old.

A leak is the only reason you would need to add refrigerant. So if your ignoring the leak in your ac system, you can be cheap and grab a can and periodically recharge the system instead of fixing the leak.

That leak in your ac system is also leaking out the refrigerant oil for the compressor too...

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