If you haven't already identified it in so many words, the purpose of the exercise is this:
To demonstrate your skills and capabilities in the most concise way possible.
There are a lot of things going on in this statement so I'm going to break them down into bite-sized chunks.Demonstrate
Demonstrating is more than just saying, 'Here it is!' It requires you to 'convince' and 'prove'. These are basic communicative skills and if you don't acquire them, you are at a major disadvantage in this business.Skills and capabilities
You might well be capable of making a very nice dinner for two but that doesn't necessarily make you a chef. In Web design, you might be quite happy to produce a logo or rollover button but without knowing 'why' it works, you can't really call it a skill. A skill is something that you can do better than most other people. It is a unique ability that makes you a valuable resource to a client or employer.Concise
People are busy and don't like to have their time wasted. Not only do you have to convince your prospect that you are the best possible person for the job, you have to do it in the shortest possible time. Busy people will resent you taking up any of their time and will look for the first opportunity to dismiss you. Initial impressions are very important!