Applying to become a British national is the final step in the immigration process. It isn't compulsory to become a British national, but after living here for a number of years and making the UK your home it is an option.
Some people who have never lived in the UK can qualify for British nationality if they have a parent born in the UK, or if they have a parent with British nationality who lived in the UK for a period of time. Some people are already British nationals, and entitled to apply for passports, but aren't aware of this due to their complicated circumstances.
While the requirements to become a British national may seem straightforward, applications can be complicated and expensive. If an application is refused you do not get a refund of the application fees so it is important to get it right first time.
Even once an application is granted, it is necessary to make a separate application for a British passport.