Freighter question

[stilton]

Older passenger aircraft that have been converted to freight usually do not have ‘cabin lining’ on parts of the overhead leaving ducts / control cables etc exposed

I imagine this was done to save weight yet newer conversions I’ve seen all have a liner installed around the interior leaving no space uncovered

Is there a reason for this beyond aesthetics ?

[Webby737]

I've never given it much thought but I would assume it's to do with fire & smoke reduction and prevention.