Want to flash a custom ROM on your phone or perform other actions using the fastboot tool? Then you’ll need to plug your phone into a computer with a USB cable… for now.
But Android Police notes that there’s plenty of evidence that Google will add support for using fastboot over a wireless connection soon.

That means you may be able to install custom ROMs, flash system updates or root patches, or backup your phone without physically plugging it into the PC running those commands.
Here’s how it would work: you’d be able to reboot your phone or tablet into the bootloader, and connect to a WiFi network from within the bootloader. Then you’d run fastboot on your computer and connect to your mobile device using a new fastboot tcp (Transmission Control Protocol) command.
There’s just one catch: right now there aren’t any phones that let you connect to a WiFi network from the bootloader. But Android Police received photos of a 2015 Motorola Moto G smartphone running an as-yet unreleased bootloader that does have WiFi support. Add that to the recent Android Open Source Project commit adding support for TCP, and it sure looks like Google is working with device makers to enabled support for fastboot over WiFi.
Among other things, this could make it possible to flash system updates to devices that may not have USB ports at all, like smartwatches or smartphones that rely on wireless charging.