Writing a first draft really shows up your strengths and weaknesses as a writer. Have you ever noticed how some parts come easily? It shows where your writing is strong. For instance, when I write dialogue, it seems to flow naturally. Later, I probably won't need to do much editing of the dialogue (unless it relates to the plot, of course). I'd say it's one of my writing strengths.
On the other hand, I'm not so good at establishing the setting (among other things). I've been writing so fast, concentrating on bigger things, like the story, that my characters are practically floating in the middle of nowhere. Not always a bad thing, but if your character is in a market and suddenly walks into their bedroom...well you can see how that could cause problems for the reader. My critiquers often point out that I need to add more setting details. Sometimes, they say my settings are too much of the same and need variety.
Have you ever thought about your writing strengths and weaknesses? It's useful sometimes. If you're aware of your weaknesses, you'll have a great starting point for what to tackle when it comes to revision. And it shows where you can do more reading or exercises to develop your writing skill.
WIP update: In case you're wondering, I'm still plodding along with my first draft, with about two chapters to go. Then I'll have to go back and make sure my characters aren't floating around with no sense of time or place.