Variables affect the world in long time frames. The influential Harvard study on human happiness[1] could only have been conducted over the course of a lifetime. Check out The Long Now[2] for a group of people evaluating variables at the 10,000 year time scale.
Even if the study doesn't live as long as its name states, a clear mission is useful because it guides people's decision making. A researcher in this should be optimizing her efforts for the 100 year timescale and not for the ~5 year one.
But I am not sure how saying it is a 100 year study really changes that? Sure it might be better than a 5 year study, but is it better than one that is simply ongoing? Or how about opening a specialized department focused on this type of thing? 100 seems like an arbitrary number that was only chosen to make this seem important. "Stanford to host ongoing study on artificial intelligence" is just a less interesting headline.
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.
What about Stanford isn't a marketing gimmick? I recall signs along 101 saying something meant to be condescending like you too could get a degree from Stanford, if you paid to do an n-years master of whatever. Sounded vacuous.
