Day is the default duration on most platforms. Unfilled day orders are cancelled at the close, so you start each session clean. The alternative is good-till-cancelled.
Day orders suit active trading where every order is placed with a specific session in mind. Note that in extended-hours some brokers treat day orders as regular-session-only.
Example: you enter a limit buy at $99.50 at 10:00 a.m. The stock never trades below $100. At 4:00 p.m. the order is cancelled without any action from you.
Related: good-till-cancelled, limit-order, extended-hours