When you're in the thick of any writing stage, from plot to polish, it's easy to lose your way. A critique partner is the one standing on the mountaintop watching you make your way up a wooded trail. She's got a bird's eye view and a walkie-talkie to guide you when you're stuck in a valley where you're convinced your only escape involves scaling the cliff face.
From her vantage point, she can tell you you're on track: full steam ahead, you're clear to the happily ever after.
She can direct you: Turn left at the next plot thread and follow that past the next two character traits--they'll take you straight into revision hell.
She can look out for hazards: Follow the main conflict for another 150 pages, but watch that cliff hanger at around page 75. One wrong step and you'll fall to your death.
She can see options: In about 3 chapters you'll come to a fork in the plot. You can take either path, they will both take you to the same ending, you'll just have different plot, conflict and character along the way. The right path is mostly downhill, but the character is rather dry. The left path has a bitch of an incline, but your pacing will kick ass.
I'm lucky to have the BEST critique partner on the planet--Elisabeth Naughton, who guided me along a jungle-worthy trail for hours when she had her own deadline looming.
E, you're a keeper.