One of the many invaluable things a critique group can do is point out what isn't in a manuscript that should be. More than those nagging questions (what happened to the donkey on page 7?) I'm talking about things like missing elements. Believability. Conflict. An arc. A resolution. If you write without an outline (as I believe most fiction writers do) it is possible to write a story without a very important ingredient. It might look good, but there's a missing ingredient. Think chocolate cake without sugar. Leaves a bad taste in your mouth, you know?
On the other hand, a critique group can also consider what you've (perhaps purposely) omitted and give you an honest assessment regarding its relative necessity. As a story sweeps the reader along, a lot of little details can and should be left out. What kind of shoes the main character is wearing, what he had for breakfast, and whether or not he crossed at the light or brazenly jaywalked.
If you, the writer, have glossed over a detail that needs clarification, however, it's the crit group's responsibility to call you on it. (You need to explain how the donkey got from Milwaukee to Martinique.)
And yes, it will sting. That's when you say "thank you," and put the manuscript in a drawer, figuratively or literally, for 24 hours. (Of course I recommend you save it to your external hard drive; don't get me started on back-up issues.) Then, get your red pen, and get the donkey home.