What is the significance of the DNC or RNC keynote speaker? What is her/his job for the party?

The DNC or RNC (Democratic National Convention or Republican National Convention) speaker's primary task is to defend the next best prospective presidential candidate for the House.

Explanation:

The speaker does so by giving a keynote speech on the prospective candidate outlining his/her virtues and proves his/her stance for the same. A DNC or RNC Speaker's job is to ensure that through their speech, they are able to make a political party's immediate candidature a suitable one. They are often tasked with the job of ensuring people getting moved and swayed towards the party's candidate resulting in their valuable vote of confidence.