It’s a replicator. The transporter waveguides at the back are common in Intrepid-class replicators. You can see a smaller one in Janeway’s quarters.

Under the replicator is supposed to be an equipment storage locker (according to the Star Trek Fact Files and the USS Voyager Illustrated Handbook), so it’s likely for creating extra away team equipment. It may also be for food - we see a food dispensing slot in the USS Enterprise transporter room in TOS: “Tomorrow is Yesterday”.

I’m struggling for a non shitpost response to be perfectly honest. Voyager was a science vessel… so Perhaps some sample return with quarantine?

it's the pattern buffer. Neelix tries to touch it in some very early episode, and Tuvok intimates that doing so might kill him in some terrible way. it's not unusual to see technicians fiddling with it while in the background of scenes.

One possibility I just thought of is the 24th century version of a vacuum tube system, but if that were the case I would expect them to be on key locations all over the ship (engineering, sick bay, the bridge, ready room, etc.) and I don't think there are any others shown on the ship.

Wow good eye. Without looking too I depth, maybe for emergencies if the main is being used?

The old method of the site to site transport was to send them to the transporter room then out to the destination, but at some point that wasn't referenced anymore. I assumed this was the component that automated that process but without having to rematerialize at all, my same head-canon for the hidden back room on TNG(?).

Is that not a replicator?

It’s a replicator.

My best guess is that it's either for samples from the surface (though we have seen multiple cases where those were beamed directly onto the main pad or in the cargo hold)

OR

possibly a tools or weapons grade replicator.

The transporter is a vital component on the ship like the engines and requires constant maintenance so maybe they figured it was easier just to have something like that in there to make quick/easy access for replicating the tools they needed when/as they needed it instead of lugging it around.