I'm talking about in HG/SS
Firstly, you have some money saved up so you can use it if you needed to buy something but don't have the cash on hand.
But also, she will buy things for you that you can't get in the games normally. As far as I know it is only the "elemental berries", the ones that halve super-effective attacks like Yache Berry etc. You can't get those in HG/SS.