You will rarely write a perfect document that doesn't require any editing. You will almost always want to insert a word or two, change a phrase, or move text from one place to another. You can edit a document as you create it, or you can write it first and then revise it. Or you might want to edit a document that you created for one purpose so that it will serve a different purpose. For example, a letter from last year's marketing campaign might be edited to create a new letter for this year's campaign.
Inserting text is easy; you click to position the insertion point and simply begin typing. Any existing text to the right of the insertion point moves to make room for the new text.