The issue with the World Champion title is that it is "ephemeral" for want of a better word

 Boxing is probably the sport where this is seen the most, and the Heavyweight division being the most extreme example 

With something like tennis there is no "World Champion" - there are tournaments - and everyone see Wimbledon as being the One they all want to win 

Similar in Bodybuilding where Mr Olympia is seen as being the One title everyone wants

 There is no World Champion in Tennis or Bodybuilding 

With boxers they fight each other and earn rank, move up the Top 100, Top 10, etc 

The Rocky  film has the example of Clubber being the Best Boxer in the World, the Number One who got that rank by fighting all the others, all the "contenders" 

Yet the elusive title of "World Champion" belongs to Rocky, as he won it from Apollo then defended it against 10 challengers 

The way the accusation of "setups" and how Mickey explains them as "hand picked" goes to show how "unreal" this title is - what does it actually mean to hold this title when it is separate from the "real" world of the ranked boxers 

Rocky is called a "paper champion" by Clubber as his title exists only on paper, only in theory

 For Rocky to fight Clubber here means two things: 

Clubber has already beaten everyone else and can now beat Rocky and earn the World Champion title and belt (which happens in the first match) 

Rocky can beat Clubber, the Number One fighter, the man who has beaten everyone else, and prove he really is deserving of the title (which happens in the second match) 

Beating Clubber makes Rocky "Undisputed" 

In the real world, the boxing titles, in particular World Heavyweight Champion, ARE "disuputed"

 Mike Tyson was an example of someone who fought to unify the title, win the different belts by beating the various men who held them, to become Undisputed 

WBA

 WBC

 WBO

 IBF 

The Ring 

(let me know if there is one missing) 

at any time you could have five different men, all unbeaten so far in their career, holding one of these belts, who can claim to be Undefeated World Champion 

We have seen fights where someone has one or more belt, had a fight that was "sanctioned" by one body and not another, lose two of his belts, but keep the third - no longer Undefeated or Undisputed but somehow still World Champion

 Like a game of conkers 

Which makes you wonder what they really fight for and why they really want it

 Of course a truly genuine Professional sportsman competes for the money, how much they get paid - which would be a good explanation - "I don't really care about their paper title - I just get paid more for a fight while I carry it and defend it" .