Why would a smart person keep at least $100 balance in their checking account at all times

One of the major reasons to keep at least $100 in your checking account is to actually cover for unexpected expenses, and also to avoid bank charges or fees for either an overdraft or a returned unpaid transactions.

It could also come handy if you get stuck somewhere, or if you need to take a taxi after having dinner with your new date, and your debit/credit card is on $0.00, this could save you the ultimate embarrassment by just transfering money from checking to your savings account.