In addition to that, a side driver helping «*directly*» or «*indirectly*» his lead driver is not always visible by the points taken to other title challengers.
In terms of drivers having been a good help for his lead driver, one of the 1st name coming to my mind (in addition to Sordo 2008 in Catalunya) is Evans in 2018 with M-Sport. Without him giving better road position, giving some points, worsening Neuville’s comparative road position to Ogier after the Belgian’s road position + the Catalunya stuff, I’m not sure at all Ogier would have won the title.
We can also evoke Burns’ 2001 title with a bit of help from Martin and Solberg in Corsica.
When the fight is tight for the title, a good side driver in the team can always be an important help for one thing or another at one moment.
It is not absolutely necessary to win (Loeb or Ogier could have won the majority of their driver titles even with Serderidis or Bertelli or Al Qassimi as side driver in Citroen; we can also quote Meeke and Latvala who were quite inefficient during the season where Tanak won his title, at another level than Serderidis for sure) but sometimes, it helps.
However, it’s difficult to know which side driver will be effective because it’s a lot a question of opportunity of being in the mix at a given moment (and not a question of just being overall good even if it improves the probability).