DNA is the master copy. In most circumstances you only want one copy of it sitting around at any given time. It has developed to become sturdy and resilient to damage, and it is always under constant repair and error correction. It is also very long, and has the ability to be spun up into condensed packages for deep storage when not in use.

RNA is basically just a photocopy of DNA. Stuff all around the cell needs to use the DNA as instructions to do their tasks, but not everything can be swarming around the DNA reading it all at once. Instead, special proteins periodically "scan" the DNA and "photocopy" it to RNA. RNA is built similarly to DNA, but it is very short, and its structure makes it much more temporary. It lasts just long enough to leave the place where the DNA is stored, make it out to something that will read its bite-sized instruction, and then it disintegrates back into pieces that can be recycled to make new strands of RNA.

You can think of it like having one master copy of a very fancy and expensive book, that everyone in a company needs to read from from time to time. But instead of letting everyone mass around the book every time they need something, you have some employees occasionally flip to certain pages and photocopy them, and they send out photocopies to everyone. These photocopies are read a few times, thrown away, and then the paper is recycled.