Brokers work for you and not the bank. If you go into a bank, you only have access to their products and niches. So there is a good chance that you will not get the best deal or most suitable loan that you require.
By using a broker you have access to a large panel of lenders, therefor we can choose which lender suits your needs best.
Whether its for a purchase or refinance to a better rate, a broker has the tools and software to search most banks rates and policies at their fingertips.