Using Feedback Cycles
Peer reviews are very important. They let you know what others think of your writing and give you feedback on whether the message you're trying to convey comes across. Another point is that often you're so involved with the topic of your essay that you completely understand what you're talking about, but miss things like explaining the terminology you're using, or something seems clear and understandable to you, but to others it's confusing. That's where peer reviews come in handy.
In ESL, all the students have different strengths and weaknesses in the English language. Giving them a chance to peer review each other's essays gives them a chance to improve their reading capabilities and sense-making strategies, while also giving them a chance to see that, perhaps, they aren't the only one struggling with certain aspects of the English language. It creates a certain comradeship as they get to know each other better through their writing. By learning how to help others, though, they will also learn to help themselves. Because by noticing and identifying errors in each other's writings, they can learn to do the same in their own essays.
When teaching writing to ESL students, I plan on incorporating peer reviews into the curriculum. They'll write an essay, exchange it with two of their peers, then revise it, and then I'll give them feedback on it, leading them to revise it again for the final draft. This will help them get in the habit of revising multiple times, and of listening and reacting to feedback. I will also insist that they write the essays on the computer using a word processor, which will give them instantaneous feedback on spelling and grammar, which, evidence has shown, helps ESL writers improve their English immensely in comparison with writing it all by hand three dozen times.