I don't advocate you pay a penny toward your MS, not until you've been through several rounds of alpha reader edits, critique partners' feedback, beta readers and more beta readers.
But once you're finished with the MS (see Kate Messner's definition of that -- I like hers a lot), it pays to have a pair of professional eyeballs proofread the MS.
Why?
Because they'll see things you miss, quite frankly.
I can't spell embarrassed. I misspelled it again, just now, but the auto-correct caught it. In a Word doc that's 84K, it simply states: "You have too many errors" to find and identify them all. It gives up.
You can't.
You've read the Amazon reviews: "This was so full of grammatical errors I couldn't continue reading."
Mine was a bit better than that, but even when I was a newspaper reporter, I relied -- yep, relied -- on copy editors to catch those kinds of mistakes.
Now, I got lucky and won -- yeah, I know, right? -- a full proofread of my MS from the amazing and talented Weslee Imrisek, at Payge White Beta Reading.
Weslee caught so much I'd missed. I apparently love ellipses, incorrect em-dashes, can't spell embarrassed, and don't know when or how to hyphenate. This makes sense.
As newspaper editor, I won't say we NEVER used these forms of punctuation but...we certainly stayed away from them, sort of like the plague. Or COVID-19. The one we did use, hyphenation, I always had my handy AP Style Book nearby.
But there's no AP Style Book entry for dragonslayer -- one word? Or hyphenated -- dragon-slayer? Or two words, dragon slayer? What about dragon scale armor? Or dragon-scale armor?
Weslee was fantastic, patient and encouraging, to boot. Weslee went over the entire MS twice, in a timely fashion, and now it looks professional as I'm sending it out to agents to query. I've already had two requests, which I'm convinced is in no small part due to Weslee's attention.
And I have to thank Fraser Weir, self-published author of More Things in Heaven and Earth, and founder / administrator of Facebook group, Author Critique and Beta Readers. It's a fantastic, supportive group for authors looking for feedback on their MSs. He ran the FB contest, and I can't thank him enough for being so supportive of new, budding authors.