I've searched around, and I can't pinpoint this exact issue on Stack Exchange (sorry if this is a repeat question). I'm not certain if this qualifies as a middle-of-run outlet, or a half-switch, or what the purpose of this setup is.
Anyway, my house I just bought was built in the late 60s, so I've been replacing all of the outlets. However, I uncovered one that has three combined neutral wires pigtailed out to the top half of the neutral side of the outlet, and three hot wires pigtailed to the lower half of the hot side of the outlet.
Why was this done? Is it ok to replace the outlet using the same setup, or should I remove the pigtail? Why are there three neutrals and three hots instead of two?
Here are some photos: