I had always felt that this description of Elizabeth was put here merely to explain Anne's situation. Anne was a reactor (rather than an actor) for much of the novel. Anne had to leave her house because of Elizabeth & Walter's extravagance. She suffered a loss of bloom because of her disappointment in Captain Wentworth, which happened because of E&W's disapproval. She was given no voice in her own house because of their hauteur. Just like the beginning of Mansfield Park, which starts with the description of the three Ward sisters rather than the heroine, Persuasion starts with an illustration of the atmosphere in which the heroine resides. Perhaps another author would start with the heroine and then explain the "back story" as it was necessary, but Miss Austen is a bit more methodical.