External serial SRAM is useful for designs that require more RAM than the designed-in microcontroller can offer; in applications where intensive functions like math algorithms, data buffering, or audio/video processing occur; or in designs that need large amounts of temporary storage. Buying a larger MCU with more RAM space is one solution, but usually results in a more expensive design and longer time to market. Another solution would be to add external parallel RAM, but this adds a larger package size as well as requires more microcontroller I/O pins.