There was some discussion on another site about whether you needed to have a proposition or not as part of the creative brief.
So....here's my humble opinion.
Ad strategy normally starts with working out the brand problem. Once that's sorted, you work out what you want to do to solve the problem.
That's normally a course of action that means getting consumers to think and then behave differently. On all the products I've ever worked on it means an outcome that sells more units.
For me, a proposition is what you propose to say to the consumer. This proposal should be based around a functional benefit &/or how you want them to feel. And it's designed to get people to think and act differently to overcome the problem.
So if you don't have a problem, then you don't have to say anything.
And if you don't have to say anything, you don't need to advertise.
Brand problems that require a communication solution need a proposal.
Otherwise you risk a piece of communication which might resemble something like this....