My very basic understanding is DNA gets transcribed into RNA by effectively replacing thymine with uracil, then RNA is translated to make proteins. Looking at it from an engineering perspective, it would appear that, while the RNA to protein translation is non-trivial, the DNA to RNA transcription process is rather redundant: As it's just a simple replacement, why not start with RNA in the first place? My assumption therefore is that there is something about the RNA molecule (e.g., too reactive, too unstable, etc.) that requires it to be put in "cold storage" as DNA.
I don't have a background in genetics or biology, so could someone ELI5 why DNA is needed?