This is my take on it… any other opinions / knowledge on this subject are welcome.
An aggregator acts a bit like an agent, putting bands music into other people’s libraries on behalf of the artist. The aggregator takes a percentage on sales but often does not even sell from his own site.
This can be useful for an unsigned band because it means that the aggregator takes care of getting their content into all the relevant online retailers.
An aggregator has the contacts and knowledge of how to get bands / labels music into the right online retailers and can generate a revenue without having to develop and promote an eCommerce site of their own.
Aggregators tend to put a band’s / label’s music anywhere it has a chance of being sold, whether a big download store such as itunes and Tunetribe or in any of the numerous independent websites. Aggregators have been known to provide catalogue to ringtone sellers and other media industries.