New hardware is usually built to spec, or at least pre-configured by your vendor. This generally adds a lead time of 2-6 weeks for delivery, whereas used hardware is usually available for same-day installation. If you need network hardware ASAP, buy certified refurbished.
Known hardware and technologies require less training to use, and reduce the chances of your IT department making a mistake in implementation.
Quite often, buying one piece of new network hardware will necessitate buying more hardware that’s compatible with it. Used hardware or even better, certified refurbished, makes it easier to swap out a single piece of equipment without setting off a chain reaction of upgrades.