A peripheral is dedicated hardware that performs a specific function autonomously, letting the CPU offload work and run in parallel. If the CPU had to bit-bang a UART, toggling a pin at precise intervals to shift out each bit at the right baud rate, it would be fully consumed by that one task and couldn't do anything else, and the timing would be at the mercy of interrupts and code paths. A UART peripheral shifts the bits out on its own at a hardware-precise rate while the CPU does real work, signaling back via a flag or interrupt when done. The same applies to timers (count ticks without CPU involvement), ADCs (convert analog autonomously), and DMA (move data without the CPU touching each byte). So peripherals exist for parallelism, timing precision, and power efficiency, the CPU configures the block, then sleeps or works elsewhere while the peripheral runs. This parallelism is the core reason a tiny MCU can juggle many real-time jobs at once.
Embedded Systems Fundamentals · Interview question
What's the point of a peripheral, why not have the CPU do everything?
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From the lesson
Inside an MCU
The block diagram demystified: a CPU core, flash and SRAM, and peripherals wired together by internal buses, and why peripherals working in parallel with the CPU is the whole point.