Ryan Parent spent most of this season as Philadelphia's 7th or 8th defenseman. He doesn't play tough competition and tends to hemorrhage shots against. He does play a role on the 2nd PK defensive pair. But that's basically it. He enters his RFA years with an established value not much above the NHL minimum salary.
Dan Hamhuis was Nashville's third defenseman behind Ryan Suter and Shea Weber. Hamhuis' role had decreased across the board as the Predators' younger pairing improved. He's due for a 30-40% raise over this season's $2.5M salary and this deal gives Philadelphia nothing but a negotiating window. That they sent Parent over means they think that window is worth less than $500k to them.
Dirk Hoag has many more details at On the Forecheck.