rna is a completely different molecule, with different properties and used differently by the cell.

For one, rna is less stable than dna, it will fall appart quickly while for dna that process takes millennia.

But more importantly various cell machinery will only accept dna or rna for their respective functions. The cell can put dna into a nucleus and still let rna move outwards to ribosomes for example.
Ribosomes are built from rna and incompatible with dna, so there is an isolation ensuring dna can't get "executed" unintentionally. There are also a large range of gene regulation mechanisms along this extended chain.
Since evolution kinda codes randomly with whatever it has, this duplicated mechanism of storage will be used all over the place. For example here you can give short double stranded rna a suppression effect while keeping short double stranded dna free for a different purpose (like maybe crispr).