My money is on some building plumbing (e.g., fire standpipe emergency drain valve or somesuch) that code requires to be accessible for once-in-a-blue-moon maintenance and cannot be sealed inside drywall. In some more industrial lofts, that would be left exposed, but if you’re wanting fewer exposed pipes, things like doors are a reasonable solution given that special purpose access panels can be quite ugly. Depending on what is adjacent to that bit of wall in the loft upstairs, a door in that location may have been the only viable option.