This habit of hers seems to have come from being brought up as the child of a Depression kid. It doesn't take a genius to realize that someone who grew up in the dirty thirties like Marian came away with the belief that saving items that the younger generation think simply take up space for no reason is a clever thing to do because lean times might come again. The problem is that as always, Elly copies a behaviour without understanding why. Since she doesn't really understand real privation, she's simply going through the motions and saving up things she likes to have handy instead of being like Marian and saving things she doesn't actually like but will need anyway.
This is why she's stupid enough to get caught short of party supplies when children's birthday parties roll around. Since she can never seem to remember that she'll always have to have about two or three extra settings owing to spillage, breakage or droppage, her habit of being generous to her friends leaves her in the lurch. Then again, nothing she does works as well as when Marian does it; thoughtless imitation never does.