He is, after all, the expert on driving Ferraris fast.
There isn't anyone at Ferrari with more expertise for driving them fast than Fernando Alonso. The two-time F1 World Champion has a unique insight into how technology trickles down from top-tier racing. He's obviously a valuable asset to the company in developing supercars for the road. It is therefore not at all surprising that he has been involved in the testing of the new LaFerrari hybrid hypercar, the successor to the old, yet highly desired Ferrari Enzo.
Ferrari has put out a video of Alonso testing the car on the track, and it must be quite a machine, as even he seems to be concentrating pretty hard.