For example, I have OCD (real, diagnosed, not the bs "omg im so ocddddd"). I have extra time accommodations because I have to spend time dealing with my OCD symptoms. With treatment, they tend to fade into the background. They re-emerge only in high stress situations. I would seem like a perfectly normal student in class, but then clearly start struggling with these symptoms if you watched me take an exam. Consider, many other students you teach may have these same experiences.
Success as a lawyer often requires the ability to handle a certain amount of pressure. Timed exams are one way of screening for that ability. But it's by no means a sure-fire predictor of success: Legendary trial lawyer Joe Jamail [0] flunked his first-year Torts class at UT Austin [1], yet went on to become a billionnaire.
[0] https://en.wikipedia.org/wiki/Joe_Jamail
[1] https://abovethelaw.com/2015/12/r-i-p-to-a-billionaire-lawye...
¯\_(ツ)_/¯
(I never met Joe Jamail, but by reputation there was a lot about him that I didn't especially admire.)
But it's beyond rational dispute that Joe Jamail was one of the most successful trial lawyers of our era.