...So again, what good is a spare?....
These main transformers, are very hard to replace, I think they come from overseas suppliers, and take months or years to have an order filled for replacement.
Smaller transformers, for down line outages, are already kept in reserve already, because they do go out more frequently. Also you will need the larger transformers operating to be able to locate outages downline. They are the centerpoint of electrical distribution.
Thus, these spares are best stored for instances of deliberate sabotage or attack.