Maybe you did it yourself and were not aware of its consequences.
The IDE highlights this unit and classifies it as "unused". In fact, it does not contain any code and thus there is nothing which can be called from your project. That's true. But it is not "useless". It adds all fpspreadsheet reader/writer units to its uses clause, and this has the effect that their initialization code is executed and registers the reader/writer classes in your project.
The IDE hint about "unused" units, however, is correct in most of the cases nevertheless. For example when you added a component to a form but later deleted the component again. In this case the unit of the component remains in the uses clause and really becomes "unused".
So think twice when you remove an "unused" unit.