If you choose to listen to the publicity, they did it to honor the memory of Ahmet Ertugen, who was quite instrumental in developing their career.
If you choose to actually think for yourself, they did it for the greenbacks. Plant did a CD with Allison Krauss (I've listened to some samples of it, not all that terrible for what it is) and touring some with her. Jason's still living off daddy's legacy. Who knows what John Paul Jones is doing, but then he always was one to stay out of the spotlight. And Jimmy's most likely continuing his slow descent into insanity.
This one show has put their music into heavy/heavier rotation in classic rock radio around the world for weeks, hell even Sportscenter has been using it for bump music. They get paid for that too ya know. The buzz this has generated will sell CDs as people go complete their collection or buy the box sets or whatever. But I do agree that they probably waited 15 years too long to do something like this. Live Aid appearances back in the 90s drew monstrous crowds. If they were to come within 100 miles of me and if I had the small mortgage amount to afford the tickets, I'd be there. It's rock history.