Some bacteria have an R plasmid. These can be transferred from one bacterium to another through horizontal gene transfer. Why are these R plasmids so important?

R plasmid contain genes which are necessary for gene transfer.

Explanation:

Circular piece of DNA that exists outside (in the cytoplasm) of the main bacterial chromosome are known as R plasmid, they replicate independently of chromosomal DNA.

R plasmid play very important role in horizontal gene transfer, they contain genes which are necessary for gene transfer.

R plasmid transfer from donor to recipient cell.