I think when they talk about a tank they mean an air reservoir which gives a more steady stream of air rather than the 'pulses' of air coming straight from a compressors piston. As Glenderg says, the newer non-reservoir compressors have probably overcome this problem though. I remember a few years back, when I was researching which compressor to buy, I read about one guy using a normal cheap 12V compressor to pump up the tube from a car tyre and he then used this as his air source for spraying. A cheap and cheerful reservoir I guess!