Is it necessary writing cover letter when applying in the job? and what is a prefect cover letter can i write when applying in legal jobs?

I would like to believe, Covering letter is almost as important as a resume/CV. A cover letter would do the same job as a soup/h'orderves would do on a nice dinner - stimulate the mind of the evaluator. A Cover letter is a sales pitch, at best it could act as an advocate for the applicant. 

A well written and nicely composed cover letter would explicitly convey interest in the position applied, articulate a brief understanding of what the company is looking for an ideal candidate and towards the end build a nice case of why the applicant would think he/she meets and exceeds the skills/attributes required to be a successful candidate for that role.

So go on, compose a nice cover letter and send it along with your resume. It has its merits.