When his return came with a new “free agent” classification, reports indicated it was so WWE could use John Cena as much as possible to boost ticket sales and ratings.
It seems that’s part of the story. According to a new interview with Complex, Cena says it’s also because with his busy schedule, he couldn’t commit to working either the SmackDown or Raw circuit, so this allows him to work whenever he’s free, regardless of the brand who’s putting on a show that day:
The reason that I wanted to be a free agent and the reason they kind of granted my request is because I don't know if I'll be available for Smackdown specific only or Raw specific only [schedule].
That’s not the only reason. While the 16 time World champ’s reiterated his commitment to wrestling and WWE over and over (including in the Complex piece), even he knows he can’t beat Father Time:
... I also know that my days are numbered. I just turned 40 in April and we have so many young and talented performers, I don’t know how many years I have left. So in the time I have left, I’m gonna do what I can to dedicate to this company I just want to be able to do it the most time that I possibly can.
There you go. You should continue to prepare yourself for life without John Cena: Active Wrestler, but to help see you through, the Ce-Nation leader is gonna show up and wrestle whenever and wherever he can.