Pirelli was recently confirmed as the official tire supplier for F1 starting next season. As such, it'll need to evaluate different compounds and other information to get up to speed. To do so, Pirelli will need an experienced and up-to-date F1 driver to carry out these tests. Since most of the sport's qualified drivers are currently tied to a racing team, and given the mid-season test ban currently imposed by the FIA, even picking one outfit's test driver (like Mercedes GP's Nick Heidfeld, as was rumored) would give that team an unfair advantage.
As a recently departed F1 driver still competing at a top level (and a recent world champion at that), Kimi Raikkonen could prove to be Pirelli's most attractive option. (Plus, Kimi's running Pirelli rubber on his WRC car.) Preliminary tests are expected to be carried out on a decommissioned F1 car and/or a current GP2 car (for which Pirelli's also recently been awarded the contract) before tires are delivered to the teams for testing once the current season concludes.