As others have pointed out, AirDrop creates a temporary point-to-point Wi-Fi connection between the two devices, which is why Wi-Fi must be enabled to use it.
But, and this is an important point, your computer does not need to be connected to any Wi-Fi access point.
In my case, my Mac is connected to my home network via Ethernet. I have the Wi-Fi interface enabled, but I've deliberately disconnected it from all access points. To do this:
- In the Network settings, uncheck "Automatically join this network", so it won't reconnect after a restart.
- You might also want to uncheck "Ask to join Personal Hotspots"
- If you are connected to a network, option-click the Wi-Fi icon in the menu bar and then click "Disconnect from your_SSID" to disconnect.
At this point, Wi-Fi is on, but it is not connected to any access point and will therefore not be used for LAN communication. But the Mac will be able to use it to create the point-to-point connection used by AirDrop.