The couple’s date night was for a good cause, as the gala performance of musical benefited Sentebale, Prince Harry’s charity with Prince Seeiso of Lesotho.
I saw the sentence from this article. I don't understand the part in bold above. I know Sentebale is "Prince Harry’s charity with Prince Seeiso of Lesotho". The word 'as' here seems to be a preposition, indicating what someone or something is or is thought to be, or what function they have. But I get lost with "the gala performance of musical benefited". Can someone help to spell it out?