The cover letter is a good place to make it a bit more personal, all while backing your motivations up with evidence.
You could talk about what sparked your interest in a career in commercial law and what made you develop it further, why the firm stood out to
you, what your USP is and why it makes you fit for a career in commercial law at the firm.
This is from a few years ago but I found this
resource by Izzet Hassan incredibly useful in personalising my cover letters and adding detail to them. I hope you find it helpful!