I do a lot of work with detached, wood-framed houses built in the United States between 1850 and 1920. Almost all of them have a walk-up attic that was not finished originally. In some cases the attics are now finished and used as living or working space, but it's clear (based on the finishing materials and styles) that the finishing was done well after the house was originally built.
Most of these attics offer significant floorspace (usually at least 500 square feet), they were built using rafters (so there is plenty of open space inside the attic) and the attics are easily accessible from the other levels of the houses using staircases.
This leaves me wondering why the original builders/homeowners would not have finished their attics in order to make them spaces that you could use for living. If you have all that space sitting at the top of your house, why would you not have spent a little more time or money to make it a conditioned, finished space like the rest of the house?
Is there a functional reason why attics were not finished in old houses? Maybe it has to do with lack of insulation, for instance, so they needed the attic as a buffer zone between finished spaces and the roof? Or was it purely a cultural thing (maybe they wanted the attic as storage space)?
I'm asking this mostly out of curiosity, but also because I'm thinking of finishing the still-unfinished attic in my 1903 house, so I'm wondering if I'd be creating any functional issues by turning it into finished space.