Not only important, but when having a contract with a client, it's fundamental. It's what defines that you have done your work and should be paid. It's what de

Not only important, but when having a contract with a client, it's fundamental. It's what defines that you have done your work and should be paid. It's what defines that your team, inside some organization, is freed from some project and might move to the next. It's what defines what you'll work on: your goal is to get the project/product accepted. You won't expend resources and efforts in other things, and you know that missing one of those items will leave your checklist incomplete and will cause you more work/pain.