This is probably one of the best decision by any TV provider. MLS is going to blow up in the coming years. There is a lot of talk about US national team being very competitive in 2026 World Cup. MLS has produced stars already, like Alphonso Davies. Once the hype gets higher and more money start being spent on soccer in the US, it will become a power house.
Liga MX (Mexican soccer) ratings destroy them for the obvious reasons (the league the US's largest most passionate soccer fanbase is most passionate about), EPL ratings beat them pretty handily as well (English Premier League, the highest profile and probably best of the European leagues).
Given the money and momentum behind MLS though, there is at least a chance by the end of this deal that they have become one of the top soccer leagues in the world and Apple looks extremely clever, which is not something you can really say for a lot of leagues with actual better soccer than MLS has right now.
It is weird that Apple, one of the biggest companies in the world, now has a vested interest in globally promoting a specific soccer league. MLS is probably thrilled with this compared to their current situation where ESPN+ does dump pretty much all the games onto streaming as content, but is generally disinterested in promoting it too heavily because of their vast prtfolio of already more popular sports.