The programmer just programs MCUs, and an emulator is designed for debugging code -- that is, it is a MCU with some debugging hardware.
Are you familiar with 80x86 programming? If so, you may already noticed that these processors have an instruction subset for debugging, which allows breakpoints and single step execution -- something you almost can't deal without while writing new code. Since they have far much larger (in comparison) resources too, and loading of new code is simple, one can use a software debugger and save time instead of trial-and-error boringness.
With a few hundred bytes of RAM, and essential (in many cases) real-time debugging requirements, even a monitor routine can greatly affect the operation of embedded system. So, serious code designing is always done using an emulator.
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Tomas