Quite important, I think. Not vital, as you can get by if they don't exist, but important, certainly. Put another way, I can't imagine trying to sell a project to a membership and not using them if they were available.
Old photographs can be used to show original bunker styles, vistas that once existed that may not exist any longer thanks to tree growth, etc., and more.
Old aerials are also quite helpful (possibly even more so), especially when it comes to comparing fairway width and tree cover over a period of time. These can often be eye-opening to members and can be the final push that will get a tree management plan or even a major restoration/renovation approved.