Windows 10 is designed for PCs with unlimited Internet connections, and it normally uses as much of your download and upload bandwidth as it wants without asking. Setting a connection as metered puts you back in control, and it’s essential on some types of connections.
You’ll always want to do this on connections with data caps, mobile hotspots, satellite Internet connections, dial-up connections, and anything else. It gives you more control over your connection and prevents Windows from gobbling bandwidth. On the Creators Update, Microsoft now easily allows you to set a wired Ethernet connection as metered, too.