What exactly is the use of PSoC?
Cypress PSoC devices have blocks (PWM modules, counters, timers, UARTs, ADC, DAC, etc.) that can be configured easily by a GUI, which can speed up the development time of a project. (no need to design external circuitry, lay it out, etc.) Also, the PCB real estate reduction is a nice plus (no need for external chips for all of these functions).

Since these blocks are actual hardware modules, you also don't need to spend time writing software to emulate these functions. They can be configured to trigger interrupts, so your state machine can easily interact with the blocks.

The PSoC 5, for example, has the following blocks: 20-bit sigma-delta ADC, 8-bit IDAC, 8-bit VDAC, 12-bit 1 Msps SAR ADC, PGA, Op-amp, TIA, frequency mixer, comparator, reference, cap-sense block. This sort of hardware is above and beyond what is provided in most microcontrollers.