Yes, every part of the cover letter should in some way say 'why you'.For a cover letter, is it ok to weave 'why you' throughout the 'why commercial law' and 'why firm' section rather than doing it standalone?
In the 'why commercial law section' you should put experiences and achievements that show why you would be a great commercial lawyer.
In the 'why the firm' section you should show them why you would be a good fit for the firm.
The 'why you' section is just any bonus points at the end that you haven't put in before.