Depends on the setup. Maybe you run everything off a raspberry pi and can't afford to transcode 4k, so you have a separate 4k library for local users only. I could also see wanting to separate the volumes when you have multiple servers attached to a single NAS.
IDK, I don't personally bother with 4k, but I imagine it's a little more to manage if you're sharing your media out with friends/family.
In literally every rural town I've been in, there's a Mexican restaurant, and it's always the town favorite. Celebrating a birthday? You're getting some fajitas.
If you ever find yourself hungry and trapped in the boonies, the Mexican restaurant is usually a safe bet.