A Power Over Ethernet (POE) switch is a network switch that also can provide power to your phone. We recommend this in all installations.
With just an network connection to the phone, the desktop is cleaner without the need for a separate power outlet and adaptor, and the power cable. Purchasing a separate power adaptor for each phone adds around $15 per device, comparable to the cost per port of a POE switch.
The only situation where a power supply is recommended is in a home office with just one or 2 devices.