In many states, the lawyer who does not lay a factual foundation for each element of his case during opening can have a verdict directed against his client. Not in Illinois. Even though a well-prepared lawyer is likely to develop the key facts anyway, he doesn't have to worry that his case will end at the conclusion of the opening due to an oversight.
Need a case cite? Here's one: Pietsch v. Pietsch, 245 Ill. 454, 92 N.E. 325 (1910).