I think it is just a way for them to see which candidate is most likeable and the best chances to get elected. Plus it gives everybody time to dig up all the dirt on them so there are no surprised in the general.
But in the case of the Dems, it was just a dog and pony show. The Dems had zero clue that Sanders would be as popular as he was because they have zero clue of who their base is and what people really care about.