Deep sleep is how a battery-powered device stretches its life from days to years. Instead of idling with the CPU running, the microcontroller shuts down its cores, flash and most peripherals, leaving a real-time clock or an external interrupt to wake it. A sensor that wakes every few minutes, takes a reading, transmits and goes back to sleep spends almost all of its time in this state, so sleep current, not active current, dominates battery life.
The trade-offs are in what is lost and how long waking takes. Depending on the chip, deep sleep may clear RAM, so state has to be saved to retention memory or flash first. Radios need time to reconnect, which costs energy and adds latency, and a device that is asleep cannot receive commands until it wakes.
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