There are expenses to keep the lights on. A small number of staff receive modest salaries. There's a fund for capital expenses. And if the parish manages to do more than break even, guess where the rest goes? To help the poor and disadvantaged. Parishioners are encouraged to share their money and time to support the poor in other ways, as well. There are regular collections for those causes.
I'm sure there is some level of corruption and embezzlement across the church globally, just as there is in just about any large institution, but I'd say that in terms of bringing in money, the vast majority of parishes and dioceses are just trying to get by, and to serve their communities when they can.
The image of the Catholic Church being some vast, obscenely wealthy, powerful organization is good for the Da Vinci Code but doesn't really jibe with reality. The Vatican, whose annual budget is about five times smaller than that of Cleveland, rarely has revenue that exceeds its operating expenses.