Editing your text while you write is perfectly possible.
However, being able to do one thing doesn't mean it's what you should be doing.
When you write, it's always best to let the words just flow.
Judging what you put down on paper as it happens leaves you second-guessing your work.
Not only does it break your flow, it wastes a lot of time doing something best accomplished much later on during the process.
The Writing Phase During the writing phase, your job is to write.
Whether you do it from an outline or from a clear idea in your head, the goal is to take those concepts and put it into words that other people can read and understand.
What about clarity? What about good grammar? What about active sentences? While those are important, they are best taken care of after you have the first draft done.
The faster you write that first draft, the sooner you can work on "decorating" the piece.
Editing Phase In the editing phase, you read through the text and decide which errors to fix, which parts to dress up and which sections to leave out altogether.
The goal is to fashion your writing into a form that will be palatable to your readers, with the intent of helping them to understand your message.
Would you ever use a writing software while you're in the middle of creating your second paragraph? Of course, not.
The common sense logic behind that is the same reason why you'd want to reserve editing after the initial pieces is written.