According to Aunt Alexandra, Atticus will bring what to the Finch family by defending Tom Robinson?

Aunt Alexandra is the matriarch of the Finch family. As such, she sees herself as the guardian of the family's good name. Alexandra is obsessed with social standing, which is one reason why she is so critical of how Atticus raises his children. The Finches have a reputation to protect and allowing Scout and Jem to run wild doesn't exactly enhance their status in town. Scout really needs to start acting like a lady, thinks Alexandra, instead of being such a tomboy and slumming around in overalls all the time.
Alexandra, then, has a very rigid understanding of social hierarchy and where her family fits into it. It's against this background that we can make sense of her antipathy to Atticus's defense of Tom Robinson. Alexandra, like almost every white adult in Maycomb, shares the prejudices of her time concerning race. Yet social prejudice is much more important to her. She looks upon her brother's acting as Tom's attorney as a threat to the Finch family name and the family's place in the wider society. But family loyalty is also very important to her, so much so that she'll stoutly defend Atticus outside of the family:

I can’t say I approve of everything he does, Maudie, but he’s my brother, and I just want to know when this will ever end.