They pay Ronaldo well because he's not only a good player, but also a global promoter of the league.
They prefer this league because it offers good pay for established star players. Perhaps in 3-5 years, younger players will join the league as well.
The Arabs are very wealthy due to their oil resources, and if they continue like this, they will make this league one of the leading leagues in Europe.
I think Al Hilal also did quite well in the FIFA Club World Cup.
He is very famous and has massive followers on his social media and in real life, so signing contract with him is like a big marketing deal for the club Al Nassr and the league, Saudi Professional League. By expecting good marketing effects, clubs in Saudi Professional League have sent several offers to Lionel Messi but so far they failed to convince Messi.
If both Messi and Ronaldo play in Saudi Professional League, it can remind us about their golden times with classic dual competition in about two decades. It can help Saudi Professional League gaining more followers and also television rights from Europe and Argentina.
Ronaldo's value is not in the number of goals he score for Al Nassr, or the games that he win for them. His real value is in popularizing this league. Before Ronaldo joined Al Nassr in 2023, Saudi Professional League was a relatively unknown competition, with hardly any viewers outside Saudi Arabia. But now it has become one of the most popular football leagues. Also, Ronaldo was on the forefront during the Saudi Arabian bid to host the 2034 FIFA World Cup (which was successful).
It is main part of his value and contribution for Al Nassr and Saudi Professional League but I am sure that clubs want titles too. It's better if Cristiano Ronaldo can help Al Nassr winning titles but another thing is at his very old age, his contributions are limited and worse than in the past several years ago. Al Nassr must know about it and be realistic with their expectation on Cristiano Ronaldo as well.