There's usually an entire pipeline of people waiting on your work - sales & marketing, ops, support - and every delay you induce risks knock-on effects for them

There's usually an entire pipeline of people waiting on your work - sales & marketing, ops, support - and every delay you induce risks knock-on effects for them, not to mention that depending on the product, customers can be planning their own workloads around your release schedules, so you'll mess them up too. Not all deadlines are arbitrary.