Mrs Reynolds praises Darcy and Georgiana, but she also speaks about the kind of brother Darcy is. He has recently purchased a piano for Georgiana ("a new instrument just come down for her -- a present from my master") and had a room redecorated for her ("that it was but just done to give pleasure to Miss Darcy, who had taken a liking to the room when last at Pemberley").
Mrs Reynolds also says that, "Whatever can give his sister any pleasure is sure to be done in a moment. There is nothing he would not do for her."
This allows Elizabeth (as well as the readers) to see that Darcy is a considerate brother. And it helps bring in the human element into Pemberley, to make it seem like a desireable home and NOT just a grand estate.