Why is there a doctor (Dr Watson) in a mystery book (Sherlock Holmes)?

They're friends, roommates, and associates. Watson not only writes the stories, but he often helps Sherlock. He is both a doctor and an ex-soldier, and his experiences are helpful at many times in the stories.

Dr Watson is the foil to Sherlock. He gives Holmes the opportunity to express his thoughts to us (the readers) via him.

He also boosts Holmes' ego by being astounded at Sherlock's powers of deduction.

Even though Sherlock Holmes doesnt necessarily need Dr Watson he has to tell someone what he's thinking and whats he's going to do because it notifys the readers and audience of whats happening and what he is planning to do :)

actually Sherlock Holmes was a habitual drug addict , and Dr Watson was the supplier and the one who gave Holmes his injections, as Holmes was over dosing on his daily fix,and it was Doc Watson who looked after him when he overdosed, this is in the records of the time

Because Holmes doesn't act as his own biographer. The stories (with only a handful of exceptions) are told from an outsider's point of view... i.e. Watson's.

The publication you point out is a collection of reports, now not all of which have been simply written by Sir Arthur Conan Doyle. I suspect what you are studying is therefore now not Doyle's phrases.

He is the side-kick like Batman and Robin. Dr Watrson is the one who isn't the brightest one when it comes to solving puzzles but is the one who has the human element.