Why are RNA primers used in DNA replication?

RNA is used because it is easily made for a temporary purpose such as replicating DNA and then degraded. DNA is used when the cell wants something permanent, like to store its cache of protein recipes, which is what nuclear DNA is. You don't want the primer you make to stay around for forever, you only want them for the minutes while the cell is replicating its genome. So RNA is much better used, degraded and the nucleotides are recycled.