Why does the letter need to be certified?

A letter should be sent "certified" to record that you actually made an attempt to contact someone at that listed address. It also guarantees someone received the letter sent.

In the Medical field, where liability is of great concern, sending a certified letter shows as proof, that you made an attempt on X date, to contact X person specified, and that X person received the letter. This way, if anyone asks HOW you can prove you contacted the person, you have a receipt showing the proof.