Why is a cover letter important for firms
Before actually starting to write your cover letter, we think it is important to understand why you're being asked for one. We often get the question “doesn't my resume tell firms everything they need to know about me?” Your resume is, after all, an accounting of your experience and successes to date. However, the cover letter serves a distinct and critical purpose. Many (non-consulting) job postings will say “cover letter optional.” Generally, for consulting applications, they're very much required (potential exceptions are if you are being referred by a current employee or the office specifically requests only a resume). Cover letters are a way for firms to gauge that you're serious about applying to the role, as they take some time and tailoring to put together.
What is a cover letter?
The cover letter is essentially the written response to several questions you'll be asked during the interview process: 1) tell me about yourself, 2) why you are interested in consulting, and 3) why you want to work at this firm. Especially given that consulting firms recruit from such a range of backgrounds, the answers to those questions (particularly 2 and 3) are often not obvious from just a resume. In short, the cover letter makes the case for why they should interview you, and your resume has the supporting evidence.
Therefore, these two documents should really work together (make sure to check out our cover letter guidance here). Think of your resume as “what” you have done that demonstrates you will be a great consultant. Think of your cover letter as the explicit argument for “why.” Your cover letter is the narrative that pulls your past experience and future motivations (something that is not captured on the resume) together.