The World Tour Finals is one of my favorite events and should be considered a major title for different reasons.
Only the top eight players can get in. During the last two months of the year, we see players who have a shot at qualifying do everything they can to get in. Murray 2014 is a good example of that. It's a great honor, you get major exposure, and there are a lot of rankings points at stake.
It's a showcase of the best vs. the best. We get to see 15 matches of the year's top players going against each other. What more could you ask for? Sometimes we see some winners at Slams who didn't have to fight not one person in the Top Ten and take the title easily. But the WTF is unforgiving. Every match is tough and whoever wins the thing really earns it.
It's a testament to physical strength and durability. The season is long and so many players can't cope with it. We've seen so many times, Nadal for example, where the player just doesn't have the physical ability to participate in the whole season and not show up. There are players who have great ability, but their physical ability is not up to WTF standards. The WTF is a true test of a player's durability. Federer and Djokovic have amazing physical bodies that don't often break down. The fact that they were able to take this tournament so many times while Nadal was sitting at home is one reason I scratch Nadal off my GOAT nominee list without a second thought. I think that's why we see players like Corretja or Davydenko win the tournament. They were able to take the title with great play and playing at a time when the "top" players were hurting. There's no shame in being the strongest, most durable player on the tour. The WTF is a great reward for that.
The tournament is often the final stage to determine who is Year-end No. 1. There are years like 2015 where one player just blows every body out of the water, but there have been some years when the YE title came down to the last couple of matches. I remember 2000 when Kuerten overtook Safin by winning the WTF and taking the No. 1 spot. That was exciting. The WTF offers that excitement as well, something the other Slams can't do.
I'd be willing to bet that all the fans of Nadal would be singing a different tune about the tournament if Nadal had won the thing a couple of times. But that may be something we never know.