Why is it important to send a cover letter with a resume?

It's important to do so because you want to be sure it looks professional, just like it's important to add a cover to a report. It's also the first look at the resume and gives the overall look of it. It lets the job or employer know the specifics and into the resume talk more of what you do. It's like the overall information page Hope this helps. 

Is this a statement or question?

It can sometimes be called a process chart.