For those not in the UK, I can't underestimate how big (both literally and figuratively speaking!) Glastonbury is!
It's 180,000 people, with over 100 stages and tents, not just music, but comedy, theatre, arts, markets, all sorts of stuff.
It's a huge coup for anyone to play (just look at how hard Metallica pestered to headline).
PR wise, it's massive for the band, it gets so much coverage on the BBC and in the european press.
Tactically, if the band want to increase their fan base even further over here, it's a gig they should do. As headliners, they get given 2-2.5 hours, plenty of time to do the same show they've been touring.
Also, the crowd at Glastonbury is amazing. Such a mix of people, all ages, coming together for 5 days. I can't recommend it enough, it's the best music event I have ever been to, and I'm hoping 2015 with the Mac will be my 5th.
Seeing them on that iconic stage, to over 80,000 people (headline acts get anywhere from about 70-100k) would be the icing on the cake!