Why your embedded controller may not need a CPU?

In most microcontroller architectures, a "smart" CPU is surrounded by a set of relatively "dumb" peripherals. The peripherals have limited functions; usually they just convert data from one form to another. For example, an I2C peripheral basically converts data between serial and parallel formats, while an ADC converts signals between analog and digital. The CPU has to perform all of the work to process the data and actually do something useful with it. This, plus close management of the peripherals, can result in a lot of complexity in the CPU's firmware and may require a fast and powerful CPU to execute that firmware within real-time timing constraints. This in turn can lead to more obscure bugs and thus to more complex and expensive debugging equipment, and so on.
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by Mark Ainsworth, Cypress Semiconductor @ EETimes