Cam Newton is no stranger to the NFL football field or the struggles and victories that come from it. He has had injuries and losses but he has also been to a Super Bowl. He brings the perfect experience skill set to mentor younger players.
Especially if Cam was to take the backup role, he would have a lot of time to speak to other players and mentor them through the experiences they are going through on the football field. He would also be able to gain some knowledge from Drew Brees.
The experience he would gain from Drew Brees alone is worth Newton signing with the New Orleans Saints. He is a very different quarterback than Brees, but he is a quarterback that Payton would use with his running ability.
I could even see Payton using Cam as a change of pace to offset Drew Brees and to give him a little more rest. That would prepare Newton to take over the Saints post the retirement of Drew Brees.