Imagine this scenario: You are in love, have just got engaged, and shortly after begin sending save-the-dates. You think there is still plenty of time before your chosen date, so you think the planning can wait a bit longer because the wedding date is still over a year away!
Then life happens, and work gets in the way, and something unexpected comes up; this thing demands all your attention, and all of a sudden there are only three months left on the calendar before the big day. And you only have the venue booked. This realization is not a fun one, your heart will race and you will have sweaty palms.
Or maybe the two of you began planning with great enthusiasm, but slightly underestimated the amount of work. The same moment of panic occurs, where it is three months until your wedding date, and everything still has to be arranged.