Even if your assumption is correct, Barcelona's defeat to Arsenal with a score of 5-3 will be an object of ridicule for some people who hate the Catalan team. I agree with your assumption that friendly matches are nothing more than test matches to give players a chance before the official competition starts.
After all, friendly matches are not matches that are so important for all teams, let alone to play as much as possible. It's hard to expect friendly matches to be very interesting because players tend to play it safe to avoid injury, but the result of the match will be judged positively by each coach of the two teams. Next match, Barcelona will face Real Madrid for a friendly match at the AT&T Stadium in Arlington, Texas, United States.
It is true that this friendly match is nothing but a club promotion event and expands the fan base of each club, either Barcelona or Arsenal. I agree that the benchmark for preparing for next season in friendly matches is not quite right, meaning that friendly matches cannot be a full parameter to measure the quality of the club's squad I think.
Broadly speaking, this is just an event to give playing minutes to young players, by fielding several players from the main squad will attract the attention of the audience. The rest greet the fans and become a holiday venue for the players before the league officially rolls on.