When dealing with a bank, you are limited to their product line, which may not be the best product for you. They will not tell you that because it is their job to sell you their products. As well, the bank has to look out for their bottom line and at times clients suffer by getting much higher rates than the deserve.
When dealing with a mortagage broker it's much more different-he/she can provide you with a wide range of mortgage options to suit your needs, and therefore you can benefit from lower rates!
Are you concerned about fees assciated with a mortgage broker in regards to his/her services? There are NO FEES applied to a client applying for a mortgage, his/her fees are paid by the financial institution that can accomodate your needs.