There are various types of reasons for deadlines. I have seen everything from very good reasons (an external API has to be replaced before its sunset date; regulatory changes that must be met when they come in place) to very bad ones ("$manager said it should be done by $date"; when asked $manager why, the answer was "I thought it should be doable by then, but it's not actually that important").

A deadline without the information of its importance is pretty useless, unless you happen to be in an organization where all deadlines are there for good reasons.