Keeping your rough draft is a decision you need to make on your own. I keep all my drafts, just in case I need a reference. Your draft is the first initial ideas on what you want to write about. Keeping it will mean you have a starting point or reference to what thoughts went through your head on writing. It is also good for prosperity. Think of it this way, would you throw out the initial sketches of a painting prior to finishing the art? No, you would keep them for reference.
A draft, no matter which stage it is on, should always be kept. For nostalgia, at least. You can look back and see the writing process as it morphs into the amazing art that it is. A mind at work, is what you want to see in the evolving form.
Yes, a first draft is horrible to look at, scattered in thought, and mostly just rambling. No, it is not dispensable for you need to nurture your writing in order to grow. Think of the drafts as pictures of your child growing up, you don’t just throw them away on a whim. Everyone should keep their drafts so they can mark their progress as a writer.