Young talent is a key word there. Guys coming off their rookie contract are ideal FA's because they are still young (24, 25, 26 year olds). They had a rookie year, where their contributions were limited, a second year where their weight room work and film study improved their game. A third year where they really started putting it all together. I like looking at these players because they're still young, yet now much more ready to truly compete with the early learning curve behind them.
Many, also had any nagging college injuries addressed in that window, which could have affected their draft rankings or early career playing time. The next 5 years will be their best years of their careers as maturity, strength, knowledge, etc. kick in. Those early years also provide NFL level film for use in evaluations, unlike college players where the 'can they compete at this level' question is always a speculative unknown.
There will be guys (like Jerry Hughes was) that will be out there for the taking because of scheme fits, or position depth bubbles. I'd like to think from his outline, we'll see young FA players matching this profile brought in to help balance up the roster.