could it be a pneumatic door opener/closer?

Not pneumatic, but it's definitely a mechanical actuator of some sort. Any signs of ramps or other handicapped access. It may have been one that was installed and then crudely removed because some idiot real estate agent thought it detracted from the house.

How does this relate to the space.

I think it's part of a wheelchair lift.

What's the perspective from which the photo is taken?

This

@Seabornman, How do you think it might have been used in an entry? Was it an inexpensive way to make an automatic door opener?

Is there a rise at your entry that would necessitate a lift for a wheelchair to get in?

picture out of context please take two or three shots of how this relates to the entry you mentioned.