If the cover letter is optional, submit one only if it adds something useful to your application. A generic letter is not automatically better than leaving it out.

The purpose of the letter is to give the recruiter information that is not already obvious from the resume, such as motivation, a meaningful connection to the firm, or context behind your transition. Repeating your resume in paragraph form adds little.

A letter that says you value teamwork, problem solving, and impact could apply to almost any consulting firm. A stronger version might briefly connect one specific experience to why you are interested in that firm's work or office.

Write a short draft and ask whether each paragraph teaches the recruiter something new. If it does not, either improve it or skip the optional letter rather than submitting filler.

Optional means optional imo. Generic filler can be worse than just letting the resume speak.