Friendly matches are part of the matches that are difficult to predict. Just like you have said, it adds nothing to the team. Don't be surprise if the underdog win the match with their big odds. I do catch cruise with games like this sometime if I have some extra cash. I will just pick like 5 friendly matches in which I will place bet on the underdogs individually as a single bet. If just one of the game win then I still win big. Although this is risky but worth trying once in a while with little money that you can avoid to lose.
For club side, friendlies does little to almost nothing and so most times the big clubs will not stress much on friendlies so as not to sustain injury playing a friendly game or keep their players in a position where they become fatigued resulting from the stress they got playing friendlies so they rather just play and get what even comes out from the game.
With countries it's actually different because it helps with their ranking and that's why it will be common amongst countries to take their friendly games serious than clubs would. I personally don't gamble on club friendlies because like you rightly stated, the underdogs could win when you expected the nigher team to, so it's more like an almost unpredictable game although it's relatively especially when the big team have got a serious game after the friendly, they usually don't put in a their best otherwise they make sure to win while the smaller teams learn from their performance.