It’s still golf, of course, so the main activity is watching rich men with bad fashion sense hitting balls around, but the interest comes from seeing what’s one of the quintessential solo sports turned into a team competition. It’s also down to who those two teams are: the USA and Europe. While there are US teams competing in many tournaments, it’s a rarity for them to be competing in sports that are popular in the US and I can’t think of another sport in which a specifically European team competes, though I’m sure my ignorance will be corrected in the comments.
However, I don’t think the nature of the sides is what necessarily makes the competition so interesting. It’s watching sportsmen who normally compete solely for themselves having to learn to compete as part of a team that makes it interesting, knowing that their efforts are just one part of the whole. It’s also that it’s a contest purely for pride and a trophy – there’s no prize money on offer (though none of the players’s sponsors are particularly stingy), just a ?250 trophy (well, that’s what it cost in the 20s) for the winners.