If you go to your router and go to the administration section, you should be able to see a listing of all wired and wireless devices. The MAC addresses will be there. There are also any number of free wifi sniffers that will give you details about your network.
Best option for LAN devices is to pull from your router. If you don’t have router access, you can try to query the device with a dos command if you know its IP ( nbtstat -A 192.168.x.xxx ) but again, hit or miss depending on the device.
It may be that someone wants to assign a fixed IP address to a device, and for that, you need the MAC address. Or, you are trying to identify devices on a wireless network for security reasons, but all you have is a list of Mac addresses