It is! - except it's to hide the fly tower from the outside. The fly tower wasn't part of the original design. The first resident theatre company to be - the Royal Shakespeare company insisted upon one so the architects but came up with the genius idea of hiding it with a conservatory. I discovered this when working in the theatre space. I went exploring the fly tower (as you do) and opened a door at the top. I assumed I'd see some dark service corridor, but instead emerged into the warm, humid, nighttime air of a huge conservatory - it was easily the most magical architectural experience I've ever had.
I wasn't sure what you were discussing. Like, what is a fly tower? So I went down a barbican rabbit hole and found this part of a video, where one of the theatre fly techs describes the same thing as you!
The 'flys' are the enormous curtains and backdrops used in theatre productions. They are raised and lowered to set new scenes in a production. They need a large space to be raised into, above the stage--which is known as a fly tower.