What is the use of wifi in mobile phones

There are a few reasons for wifi in a phone, the main one being that in many phone plans, wifi does not count toward minutes or data bandwidth so you save money. There are also situations like where I live where cel reception gets iffy at times - I know I'll always have a connection at home when I have wifi enabled.
As far as enabling wifi on your phone, go to; Settings>>Wireless & Networks there should be an option there for wifi, select it. You may be prompted for network passwords.
Hope it helped.