The Assessment Task
Replicates what happens in the real world. It integrates knowledge, skills and attitudes just like the real world would. It is a useful, transferable task useful in the real world - otherwise why learn to do it? (I don't deny that until the IB Diploma changes kids still need to learn to sit a test mind you).
In life outside of formal education you wouldn't normally find yourself in a silent exam room. That's not real life once you're grown up. Real life contexts tend to be noisy, messy with several resources to choose from some of which are not useful at all. Time restraints (at least those to the minute) are not usually a reality.
In real life more often than not you have to work with others to get a job done. Therefore if the task you set would normally be a collaborative endeavor in the real world, then so should it be in your classroom.
The Assessment Result
In real life - after formal education - we don't find ourselves writing tests or essays (one exception I can think of is the written driving test). Instead we produce quality products and quality performances to please our bosses, partners, friends and spouses.
In the real world you know what is expected of you. If you don't know you would ask. Think for example about an architect. She asks the 'man' with the money what he wants before she goes off to design the building. We ask what the dress code is before we turn up to work on our first day. In real life we know success criteria in advance.