Note: I am writing this from my phone, so please be a little compassionate toward my errors. Pretty please?
My family and I are on vacation right now, visiting family and friends in Salt Lake City, Utah. We're having a grand time, and it's nice on the budget to be visiting people we know because we get to stay at their houses.
While we've been doing this, I noticed something. Everyone's house has quirks, weird little things that don't work right or make strange noises or whatever. This is especially noticeable in the bathroom, probably because it's such a private place (so asking for explanations can be awkward) and probably because you really have to use that room at some point.
The thing is, everyone is used to their own bathroom's quirks, so we hardly ever notice them. For example, in my bathroom, we have two toilet seats. One is a smaller version insert into the larger version. It's for our little kids when they're learning to use the toilet, so it's extra small. I'd never thought about it, though, until a friend came out of the bathroom and said, very awkwardly, "Soooo, your toilet seat is kinda small." I realized she used the kiddie seat because it isn't obvious that the other seat is under it! This is something I never thought to mention.
Staying at my sister's house, I've discovered that the pipes to her downstairs bathroom sounds like a moaning ghost when the water is running. This is a sounds she's used to but that surprised me a bit. And at my in-laws house, there's a bathtub that doesn't have a shower, which was so crazily weird to me when I first discovered it. And at a friend's house, the hot water comes out of the sink scorching. Oh, and there's the sink handle at another house that I accidentally pulled off when I turned it! Fortunately, it was broken before I came along. But you get the point -- all houses have quirks, and we get used to our own.
Now what does this all have to do with writing? Okay, originally it had nothing to do with writing. I was just in the shower desperately trying to come up with an MMW post (and also desperately trying to get the water temperature right) when I thought of it.
But then I realized actually it does relate. In our own manuscripts, just like in our own houses, we get used to the weird stuff and the stuff that doesn't work right. We sometimes barely notice it or maybe don't even see it at all! We need beta readers (or whatever you want to call them) to look at our stories with fresh eyes and see what isn't working. They'll notice the jiggly flusher or the creaky tiles when we won't.
And they can't all be our critique partners. Our critique partners have essentially been living in these houses with us, or at least visiting often enough that they too miss the stuff that isn't right. Maybe you had a scene that explained why the character acted a certain way, but then you pulled that scene out. Your critique partners still understand, but your future readers wouldn't get it. Enter the beta reader! They find and help you fix those quirks before they go to your readers.
Don't get me wrong, I have nothing against houses having quirks or unfinished projects. Such is life. But if you're planning on selling your house, you're gonna have to fix a lot of that stuff. To you it may be charming or at least invisible, but to a prospective buyer, it isn't. Same goes for a story. Do you want your readers to "buy" it? Then fix it up.