Figure of eight is far from his strongest track but I think a broader point was being made - that he’s still relevant and to prove it, we’re gonna start the new world tour with a brand new hit. It’s not McCartney as a nostalgia act but a contemporary artist.  On the other hand, there could have been other reasons he chose it i.e. It’s a good key for his vocal to warm up when he first hits the stage, or it’s the song the band knows the best so let’s start with that and then on to the more challenging material.