It is Frank's 14th birthday and he was lucky enough to find work at the post office as a lower-end messenger. The reason why Aunt Aggie buys Frank new clothes is because, as he reported to work, people laughed at the clothes that he had because they were raggedy. Frank was lucky enough that he did not need to report to work until the following Monday, which prompted Aunt Aggie to take action.
She not only buys him clothes, but also gives him money to get tea and bread. This is quite a luxury considering the overall financial situation of the family. Moreover, this is shocking behavior coming from Agnes who is a whiny, abusive, and angry aunt. However, the overall message of her actions is the old saying that "blood is thicker than water". Aggie has no real reason to hate the McCourt children other than Malachy McCourt, Sr. She is also clearly frustrated and dissatisfied with life, for which her negative attitudes come uniquely from within.
The implied message in Frank McCourt's unique storytelling style is that the moment when Frank got the job represented a hope for the future of the family; he will begin to earn wages so he will also help his family come out of misery. He also was "coming of age", which implies a right of passage that invites to bestow upon an up-and-coming young man a distinction of some sort. Hence, as the temporary caretaker of such young man, Aunt Aggue basically did the "symbolic honor" of giving Frank a helping hand so that he could succeed in this new endeavour. She gave him his first " working man" wardrobe.