Urgency, when used appropriately, can increase your email open rates. Using it inappropriately can increase your unsubscribes and spam complaints. So when you do use it, make sure it is appropriate to do so.
Urgency subject lines like “24 hours only”, “today only”, “only hours left”, etc. Help to encourage the reader to open the email now instead of saving it for later or just deleting it.
So when appropriate, inject a little urgency into your email subject lines.