What is the key problem that is trying to be solved? Empathy with the prospective user, and understanding what the problem might be is extremely important, and a problem statement helps accomplish this.
A problem statement further helps avoid coming up with solutions too early on by focusing on the 'why' part of the problem. It is also an excellent way of communicating the problem to stakeholders.
There are a number of different ways of creating a Problem Statement but the examples below are the more common approach.