I can picture Mrs. Morland, wanting to make sure that Catherine has all the necessities ("make sure you wrap up when you leave the rooms" to paraphrase) and hoping that she has done all that she could to prepare Catherine for this extended time away. Hoping that Catherine will always be polite, etc. And Mr. Morland giving her an amount of money that he deems appropriate, but also knowing that she might need more eventually. A good way to make sure she has enough but not enough to spend it all at once!