Because of separation of concerns. This will greatly help unexpected references between classes/objects.
For WPF/Silverlight programmers, think about the MVVM design pattern: separating ViewModels and Views into two distinct projects will ensure there is no reference of a View object into a ViewModel.
Another point is the build time that could be shorter as the whole solution won't be recompiled each time. Might be a good argument for your manager (but this assumption might be wrong depending on the size of your solution).
