Here is a map.
But, basically, he's there to sub any assistant referee who for some reason can't continue the game (injury, sickness, etc.), or to sub the fourth official if the fourth has to sub the main referee (for the same reasons mentioned above).
However, that doesn't eliminate the fact that there's a main referee and there are two assistant referees... so that question is correctly made.
I kept finding myself clicking "football" when I was meaning American Football and not soccer.
For instance, this ranking (https://www.theguardian.com/global/ng-interactive/2019/dec/17/the-100-best-male-footballers-in-the-world-2019) of the 100 best football players in the world has the first German at position 21, with the next one occupying the 37th place.
And another small thing: Replace Germany with Brazil for top players. I think Brazil has had by far the most top players in the history of the game.