What is it about Emory that makes you want to attend besides the obvious (e.g., Prestige, high-quality programs, location)? And how do those programs represent a particularly good fit for you? This essay is your chance to demonstrate that you've done your research on the school and that there's a fit beyond "OMGZ, it's a top 20 school."
It's really up to you to present the most compelling argument. If you're interested in psychology for instance, you might talk about specific courses or departmental concentrations that the school offers. If you're applying to Oxford, talk about how a liberal-arts-intensive environment for two years is the best fit for your long term goals.