Working with my editor has been hugely beneficial.
He saw plot holes that I was sure I had explained, but turned out to only exist in my head. A good editor is just a guide, you’re still the one writing it. And you’re not obliged to agree to all of his proposed changes, but it may give you a healthy perspective on your writing. It’s hard enough to get people to buy/download your book when you’re a nobody, but if on top of that you have gross errors in the manuscript that could have been avoided (they’re vs their), or your main character switches name from Peter to Pete, with plot holes dotted everywhere (Indiana Jones: Raiders of the Lost Ark, wink wink), the reviews will tank your book before it has even taken off. And your name will be associated with poor writing. And then it will be even harder to sell books. If you’re self-publishing, define the budget you’re willing to spend and prioritize where you want to spend your money.
If you publish on print: Don’t forget to include ISBN/barcode purchase on your list. You don’t always need one, but it helps libraries and bookshops to find your book, so don’t overlook this.