Two areas of the writing process some writers overlook are the editing and proofreading steps. The misconception is that a professional editor will take care of that work once the manuscript is accepted by an agent or a publisher. Unfortunately, most agents and publishers won't even give the manuscript a second look if they can't understand what you're trying to say because of wandering plot lines and/or bad grammar, spelling, and punctuation. Professional editors are paid to finish the polishing you gave to your manuscript before any agent or publisher got it in their hands.
     Editing is when you make sure the story flows smoothly, your characters have the same name throughout the book, the technology reflects the historical setting, and other such details. Do your character descriptions, names, and other relevant facts remain consistent? Are there any events which aren't necessary for the plot or subplots? Is your ancient Roman senator wearing pants? An editor may or may not catch these mistakes. Make sure you have your facts straight.
     Proofreading is what you do to correct mistakes in spelling, grammar, and punctuation. Unless you're an English professor, don't rely on your own skills at proofreading. Even professors should have one or two other people proofread their manuscripts. Too often, the writer is going to read what they intended to write and not what's actually on the page. You also can't completely rely on the spelling and grammar checkers on word processing programs. Slang words have correct spellings not recognized by spell checkers and some missed punctuation marks aren't noticed by the grammar checkers.
     There's another reason for doing your own editing and proofreading. If you decide to self-publish your book, it will most likely be printed exactly as you send it to the printer. Printing companies and vanity publishers have earned their money when you paid them for their services which usually don't include editing or proofreading.
     If you decide to utilize the services of a company such as Createspace to publish your book or you do no more than create a Kindle edition, your work in these two areas as well as formatting are crucial. In both of these cases, your book will appear exactly as you submit it in your PDF or other file. There will be nothing more embarrassing and destructive to your writing career then to have your book bought or downloaded only for the reader to give it a bad review for no other reason than the unprofessional quality of your writing.
     If you want a professional product, make sure all of the mistakes have been corrected.