1. Rationale initially was that we were a growing business. Leadership did not want to have short-termist mentality about bringing the wrong business through the door just to make up the numbers. Now-a-days, the no-bonus policy means that Snr Mgrs who deliver a lot of work (high utilisation, low biz dev) are not penalised vs. those who do more biz dev and less service delivery.
2. Range indicates where you are within your career. As senior manager is a grade where you could potentially spend 4-5 years (as you get closer to partnership), there needs to be room to reflect whether you're more of a "polished manager" or a "partner in waiting". Market conditions will also drive pay, of course.
3. SMs get a car allowance. From memory, it's about 5K
4. Director is a stepping stone towards partner. To get promoted to Director, you need to to be bringing in several million pounds of business a year. In general, you're not promoted to Director unless they are pretty sure you will be invited to the partnership.