They say brass can sanitize, but assuming it’s not that…

Its easier to clean and will prevent wear on the door itself.

Doors that push open are a lot more amenable to panickong people who need to get out in an emergency. You may notice that a lot of these non-latching or push-mechanism doors also swing out towards the exit.

I assume it’s an access plate to service the door mechanism.

To be able to open the door easier (let’s say, a fire) and the knob could make some jobs more harder, like moving huge stuff from one side to another. That’s all. Should be true that can be easy to clean, but that it’s a side effect of its original purpose.