It is not a requirement, but companies within batches, and within Ycombinator in general have a unique insight into the demands and pain-points of their fellow companies. This is why each batch usually has a handful of companies working on products specifically designed for other companies (e.g., MailGun, SendHub, Heroku, Mixpanel etc). Some of the companies pivot toward building tools if/when they discover their original idea isn't getting traction.
Also Parse, Apptimize, Taplytics, ZeroCater, Disqus, Optimizely, Authy, HireArt, ZenPayroll, etc.
For a startup doing B2SmallB, getting into YCombinator gives you ready access to a large network of customers. It's not just about access to investors, sometimes you can get very direct customer feedback by selling to your batchmates.
