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Of course, whether it's a friendly match or an official match, it doesn't matter for El Classico, but I think a little less importance is given to the friendly match. At least Ancelotti gave me that impression. He is currently looking for the most suitable combination for his team and so he tries a lot of tactics and players in each match. When we create the best roster rotation with these players, we can watch the real El Classico.
You may be right that in the end friendly matches are more or less only used to test players' mentality and test some strategies. Ancelotti has several new players he will need to capitalize on next season following several departures from his old players. Joselu got his chance, so did Bellingham and the others.
But whatever the results of the friendly matches during pre-season, I think Ancelotti already has a good plan for next season. I believe Ancelotti is still trying to find the right solution to his striker shortage after Benzema left, but I hope Joselu can provide him with an alternative instead of relying solely on Bellingham as an attacking midfielder.