Portable vs On Board Air Compressor?

Sorry for being so naive here but I'm still kind of confused. If I invest in an on board compressor, are you saying that it's not powerful enough to fill my tires? Only the airbags? I don't want to buy two compressors. I want one that can handle both my tires and my air bags. No I'm not going to be constantly inflating/deflating. But I'd like to have the option to be able to go on the beach when I feel like it without it being a major hassle. or put the camper on without dreading the airing up process. I just want to know if an on board is really that much more convenient.

Does having a tank or not affect the speed or capability of the compressors? The viair portables don't seem to have a tank. Does this make it an inferior product?