Now first of all, as you correctly say in your answer and question, at that point Bane was supposedly working for John Daggett, who likely brought him into the country and funded his endeavours. So part of the reason for robbing the stock exchange was to bankcrupt Daggett's rival Bruce Wayne and make his company digestible by Daggett's own company. So it was part of Bane's job for Daggett and part of the reason why Daggett even funded him in the first place.
But even beyond that, you also have a additional motivations for Bane, or rather Talia al Ghul's League of Shadows, to bankcrupt Bruce Wayne:
First and foremost it supports their exterior plan of supposedly ridding the city of the rich decadence that plagues it, best represented by billionbaire playboy Bruce Wayne as a symbol of exactly what's wrong with the city and its corrupt upper class and their financial games.
And above all that, it also supports their more secret plan of blowing up the city. If you remember, Miranda Tate founded the fusion reactor project together with Bruce and her more diplomatic attempts to get him to involve her on the results all failed, so Bruce Wayne's bancruptcy is finally a reason to let her in on the project and get her hands on the bomb she needs to destroy the city, since the danger of John Dagget getting it is much higher than the one of involving an associate that has already built a personal relationship to Bruce and enjoys his trust.
So Daggett's primarily economical motivation plays nicely into Bane's (or Miranda's) own goals and they can basically hit two birds with a single stone, doing a well-paid job for John Daggett while advancing their plan of humiliating Bruce Wayne and destroying his city.