The third and last paragraph provides means to follow up your application. You can slightly review what you have previously mentioned in the letter to provide more emphasis. You can also provide means for contacting you in case you are considered in the next phase of the application process.
Many jobseekers consider the resume as a solitary factor that can convince a hiring manager about their qualifications and their capacity to hold certain vacant positions. Believing that this alone can help them land a job, they unfortunately ignore the significance of some other things in achieving their desired career. Worse, they are totally unaware of some other factors that may actually let them outshine other job candidates.