Some application forms ask for a personal statement from the applicant – a kind of overall mission statement. The employer really only wants to know three things: whether you can do the job, why you want the job and whether you can fit into their organisation. Your statement needs to answer those three questions convincingly and, as far as possible, back it up with evidence.
It also helps to emphasise something of your enthusiasm for the job and what particularly attracts you to the organisation. Showing that you know about the company’s strengths, current challenges and latest developments can also be a way to impress.