Question: what are the main functions and roles of commercial banks and why may the banks need to be supervised? 

Answer: In my view, the first objective of any commercial bank is to maximise profit for his share holders. The second is to comply with all the rules and regulation set by the Bank of England and other regulatory bodies in the UK. Third is to use the marketing strategy to expand the business and  find new avenues to increase profit. Fourth is the customer service role which has been neglected in recent years, but some banks are try to address this issue.
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