This act was desired by most of the largest banks, brokerages, and insurance companies in the country at the time. The main reason being that people usually put more money in investments in a good economy, but when it turns bad, they put their money into savings accounts. With the new act, they would do both with the same company, so the company would be doing well in all economic times. This has in fact come true, for example with Citigroup making more income then any other company in the world.
Certain rules still exist which cause some separation between the investment bank and the commercial bank. For example, the commercial banks aren't allowed to pay commission to their employees who convince customers to also use some investment services. They are only allowed to pay them a small fee for simply setting up appointments to meet with a fincancial advisor.
See also: Financial supervision, Financial institutions