Urgent is for the few things that can't be missed. It's deliberately louder than a normal reminder.
An Urgent item alarms even when your phone is on silent or a Focus is on. Use it sparingly — it only works because it's rare.
Marking something Urgent without a time sets it for the next hour, so the alarm has something to fire against. Change it to any time you like.
Note: Urgent needs notification permission. Memento asks the first time you use it.