When You Need It
If your wireless network is lagging, especially when transmitting large files, wired Ethernet—specifically, Gigabit Ethernet—is your first and best option for getting it back up-to-speed.
Gigabit Ethernet has ten times the throughput of even the fastest real-world AirPort Wi-Fi connection, and it never suffers from interference or dropouts. You can back up your Mac, the kids can download movies, and your spouse can stream music from the family iTunes library, all at the same time, without bogging down your net.
Some network-connected devices particularly benefit from a wired connection: