You should think not only about promotion, but country laws, media contact, taxes, money collect, contract types, many of whom let the bands have money enough to start their tours, and a long etc that makes the life easier for the musicians. This is why, in the era of internet, bands still sign with the labels, and this is why the support from those labes is so necessary to survive. I won't talk about the contract types nor I will talk about the payments but, believe me, bands make much more money from the labels and the record sales than people thinks.