I can see value in having a tag set aside for the particular networking library/API used in Unity, which I think is what the unity-networking tag is intended to do, judging from its tag description:
Unity 5.1 introduced a new framework to handle client+server
networking, also known as UNet.
There are ways to do networking in Unity without using Unity's built-in Unity Networking, such as hand-rolling code or using third-party libraries like photon (which itself has a tag) — so it makes sense to be able to tag each of these use cases differently.
Similarly, I expect it would be possible for someone to be an expert in using Unity's networking API without being an expert in networking generally, and vice versa: an expert in networking may not be an expert in the Unity Networking API (or other APIs with their own tags like Photon...)
So tagging a question unity-networking/unet (I think these should be synonyms) says more than "this question is about networking a Unity game," it says "this question is about using this specific networking API"
This is analogous to tagging a question about using Unreal's node graph-based scripting system with unreal, blueprints, rather than the more generic term for that technology, graphs, to say "I need help with this particular tool, not just this class of problem"
If we find the Unity-Networking tag is being used too broadly to refer to networking scenarios not involving Unity's specific API, renaming the tag to something narrower (eg. making it a synonym of unet) may help reduce this temptation.