Powering peripherals from a microcontroller (ATmega328P)
If you source much current from the uC, it's likely for its output voltage to drop below what's acceptable for your peripheral. Changes in current consumption can also cause ripple at the output pins because of the uC's output resistance.

Your peripheral is likely to have power decoupling and filter capacitors. Charging them at the first moment will cause inrush current that can be outside current limits acceptable for your uC. It may either cause the capacitors to take a long time to charge at each power-up or damage your uC.

Anyway, it's better to have a proper power key (an external MOSFET, for example) than to power anything from the uC output except the case when the specific uC output is designed to be used as a power source.