I would make sure the cover letter is only covering things not covered elsewhere.
Consider it more like an opportunity to highlight certain things you haven’t had the opportunity to with other sections of the application, or look at the wording of the other questions carefully and think about what is missing.
For instance, the “why us” question is sometime phrased as “why do you want to train with us”. In this instance, you could focus the answer to this question on the training contract opportunity, but your cover letter could cover more about the firm generally (eg culture/values - the things that would still apply whether you were applying to a paralegal or accounting role within the firm).