One concrete example would be data acquisition. Long term, large sample panel data sets take decades to produce, but are incredibly valuable if well designed / executed.
If you are applying for funding as part of a 100 year study, you won't get continued funding unless you put in the effort up front to design the data acquisition correctly.
Another is what a project can offer your personnel. A four-year project will almost solely have temporary jobs.
A hundred-year project will be able to offer more permanent assignments. That creates less 'publish or perish' pressure. It also may attract scientists with different character traits who wouldn't be able to compete in the rat race to tenure.
If you are applying for funding as part of a 100 year study, you won't get continued funding unless you put in the effort up front to design the data acquisition correctly.