I've had times in the past when I used the Network Locations feature to have finer grained control over how to connect to the network. Sometimes this was listing networks/interfaces in different orders to have control over which network was preferred, sometimes this was removing or adding interfaces from the list.
One case was a "Wired ONLY" entry to force an ethernet connection even when there was a WiFi network present (for speed, reliability, and routing reasons) - these days this specific case can be handled by simply going to the WiFi widget in the menu bar and toggling WiFi off. I also had entries to turn on every interface (including things like ethernet over Firewire, for direct-connecting two Macs), and another entry to use most of the time that only allowed WiFi connections, thus simplifying the network negotiating process.
You might also have entries to run an ethernet connection either in a DHCP controlled environment, or using a hard-coded network configuration (IP address, subnet, netmask, gateway, etc.). Particularly if you had multiple different hard-coded network configurations you needed to use (without DHCP to query for settings, the machine couldn't tell which settings to use). Especially helpful if you had multiple ethernet networks to connect to in different locations (work/home, but could also be different labs at work or whatever) that weren't controlled by DHCP.
Another would be "WiFi with DHCP for everything" vs "WiFi & DHCP but override the provided nameservers".
These days my network scenarios are a lot less diverse - mostly WiFi using DHCP for everything, so I don't currently have need for this, but it's unfortunate to see it go. "Network Location" meant being able to have half a dozen conflicting configurations stored and switching between them with one pulldown. If it goes away and there's no replacement, then switching between two of those configurations could mean looking up notes and fiddling half a dozen settings one way or the other, entering or erasing IP addresses for nameservers and such. Seems an unfortunate choice.