Why does a provider join an Affiliate?

Miles,

The Affiliate manager receives all the money earned by the Affiliate Company and then re-distributes it among the company members.

There may be several reasons a provider may choose to join an affiliate:
1) it is the affiliate manager's headache to find projects and negotiate with Buyers - the provider can concentrate on their primary skill
2) for new providers it may be hard to get projects on their own - it is easier for an affiliate company with reputation and feedback score already earned on oDesk to sell less experienced providers under its name (because they are supported by more experience members in case of difficulties)
3) certain projects require several skills, for example, website development takes a web-designer, a developer, a copy-writer and a SEO. It is often easier to get such projects for an affiliate company who all these professionals working as a well co-ordinated team, rather than for a single provider.

Hope this gives some insight.
Regards,
Olga
oDesk