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.

regarding the law, the 15th amendment was specifically designed thinking in the rights of black free men.

even though ratified on february 3, 1870, the promise of the 15th amendment would not be fully applied for a long time.poll taxes and literacy tests would allow southern states disenfranchise african americans. only in 1965 the voting rights for african americans would be actually able to vote in the south.

in 1965, president johnson signed the voting rights act, which aimed to overcome legal barriers that prevented african americans from voting as it was guaranteed by the 15th amendment to the u.s. constitution.