Why do grad students have the luxury of
staying at Crerar after-hours but not undergrads? It seems we work just as hard and pay at least as much as them to attend this university.
A full answer requires a review of history. Sherman, set the Wayback Machine for 1984....
Back before Crerar was built, the science collections were dispersed around campus in small departmental libraries. Chemistry had rooms in Jones Hall, easily accesible from Kent and Searle. Medicine occupied a room in Billings Hospital (now the Surgery Waiting Room). Biology was originally in Culver Hall, then relocated to the B-Level of Regenstein. Likewise with Geology and Geophysical Sciences.
Eckhart Library, the surviving member of this cluster, continues to house the math, computer science, and statistics collections.
Faculty (and sometimes graduate students) needed 24-hour access to their libraries for research purposes, in case they need to consult a book or journal at night or on the weekends. They obtained a door key, usually from the departmental secretary. Mathematics faculty still have door keys to Eckhart.
When most science collections were consolidated into Crerar, it was impractical to hand out a door key to every faculty member in the BSD and PSD. Instead the Library stationed a staff member at the entry control desk to admit faculty and science graduate students who needed to do research when Crerar was closed.
Thus the Crerar 24-hour privileges were (and are) a way to let people use the library at night to do things like look up a constant they need for an ongoing experiment, or a medical article for an admitted patient.