The program referenced to here (3+2) is for a Masters degree. Since Oberlin is a college and has only a handful of post graduate programs they team up with other schools to offer the masters.
The idea is that you spend 3 years at Oberlin to get your BS and 2 more somewhere else to get your MS. This is instead of the usual 4 years for a BS and then 2 more for the masters.
The reason someone might pick Case for their masters instead of cal tech or wash U is a desire to stay near Cleve