Frankly I thought this issue was obvious and clear, but it seems writers are a little too self-involved to see it.
Too often the advice an unskilled writer gets goes like this:
"Get an editor to fix your mistakes! Oh, and polish up those first chapters in particular, because readers use samples to judge whether to buy a book or not."
Do you see the problem here?
Readers do. You hear it from them all the time.
"But I tried using samples to sort out the bad from the good, and it didn't work. The sample would be really polished and professional, but the book just fell apart in the middle and I couldn't even get to the end."
The more we use editing as a short cut, or as a marketing tool, the less effective it will be. Readers will be forced to find other ways to find good books. The power of the sample will be dead.
Remember folks, beginning writers don't know what it takes to write well. They need to learn to write first. They need to learn to do their own editing. And then, if they choose, they'll know if they need an editor or not.