There a number of theories/reasons Coples would be a nice signing for the Panthers. It's clear he would benefit from moving back into a 4-3 defense as a DE. As pointed out by Bill Voth be could also provide some insurance in case Charles Johnson refuses to take a pay cut:
The Panthers need pass rushers. So signing Coples would allow Gettleman to check that box. And, no, that doesn't mean he'd be the only guy added. Carolina could look at other free agents and this draft is deepest on the defensive line. Even if Coples signs with the Panthers, there's no guarantee he'd make the roster. If he did, it'd be a low-cost move that also wouldn't count against potential comp picks in 2017.
I like the thought of giving Coples a chance to revive his career on a defense where he won't be expected to be the star. He will at least have rising star Kony Ealy on the other side. Coming to Carolina has to be high on his wish list should they decide to make an offer given he went to UNC and is from Kinston, NC. This is where being NFC Champs should pay off, getting first dibs on the top guys available.
Coples was set to make $7.7M had the Dolphins kept him. So his value is obviously much lower than that, something to the tune of $2M to $2.5M per year (hat tip to Canadian Panther for this estimate) seems reasonable. So would you bring him in for that amount to see if he can revive his career? He would have to make the roster of course, which is not guaranteed of course.