I'm not sure there is essentially a 'purpose'. For a long time scholars paired up Homer and Hesiod as the earliest Greek poets and maintained that Homer was purely narratorial and Hesiod was purely didactic. We came to think of Hesiod's 'purpose' as telling a purely moralistic guide to How To Live Your Life. We're not sure how true that is anymore. No doubt, Hesiod and Homer were well-loved in antiquity and thus preserved. The longer they are preserved the more importance was bestowed upon them. Classical antiquity considered Hesiod/Homer the sort of hand-books on Greek divinity and morality. But you have to remember that the Greeks had a religious society with no set religious texts, like the Christians have the Bible. So they took a system of myths and drew their morality tales fom that instead. Hesiod and Homer were some of the more concrete forms these myths took. With the rise of Pan-Hellensim at the time of Hesiod, a myth of shared creation and unified diversity, as it were, became very popular. There's also a very good chance that many of the Hesiodic (and Homeric) myths were around before Hesiod himself cemented them. So asking the 'purpose' in Hesiod is like asking why myths exist at all.

The role of a bard was as both singer/composer/performer. To roam around, singing stories at public or private performances. That's not to say they are 'just' performers. They would have composed as they sung and been appreciated as artists in their own right. The idea that Hesiod's 'work is bestowed upon him by gods' is a common theme in oral story-telling. The idea being that Hesiod himself did not think up a fictional story, but was rather inspired by the Muses and acting as a mouth-piece of conduit of the Divine.