If you have 27...than you are doing something wrong?! [and playing the wrong game ]
I expected A "more" mature answer.
I had 30. Reduced to 24 later in mars in order to fit a few more barracks.
I used them because
a) seeing the supply proliferation in mars tree I needed enough to clear the tech tree there
b) I wanted to save up enough supplies to be ready for the next age if it involved another big jump.
c) i used them for the 1 hr quest for a while. not everyone can get all the spend FP quests they want. They were probably worth 60 or so quests a day to me.
I usually try to maintain about that much space availability for flexible use anyways (i.e. either non-event buildings that i can destroy and rebuild or event buildings I don't care about that I can ditch if I need the space back). If you don't well that was your choice, but you can always break out some store buildings and make space.
The one thing I'd like to recall when it comes to potentially "needing" many mars ore synthesizers is the people that yelled so loud to get the population taken away from abandoned asylum - damn you!