But for a SAC of 2 bar/minute to be useful, you need to know the cylinder size, and you have to recalculate for each cylinder size.If my SAC on a 12 litre is 2 bar/min, then I can look at my depth, say 30m (~4 ATA), and know that my pressure should drop at 8 bar/min. If It is changing 10 bar/min I am working harder than normal, etc.
However, it does make far more sense than saying SAC is based on your depth.
Knowing you expect to use 8 bar a minute during your dive at the expected depth is a useful figure.