This doesn't help ID the person (MAC tells you
manufacturer, and
sometimes model, but that's all). But it does help for blocking...
It helps to confirm that two people are the same. While a checkuser
that shows that two accounts are being used by people with the same
ISP would at best be a "likely", a checkuser that shows two accounts
are being accessed via the same network card would be a definite match
(assuming the rest of the address rules out a public computer).