Depends on the deal. Promotion and fronting of cash are the big ones (usually). They can usually also offer various development services (testing, translation,

Depends on the deal. Promotion and fronting of cash are the big ones (usually). They can usually also offer various development services (testing, translation, porting, getting special deals with platform holders). They can also act as a stakeholder for quality, advising on certain things that should be improved (as an editor would in book publishing).