When you're taking a look at your business' profitability, one of the factors to consider is whether your Principals and/or your Customers are generating a good ROI (return on investment). Being able to track the ratio of earnings to time spent on these partners gives you a general idea of the success of your sales efforts and your profitability.
We all have our favorites - favorite manufacturers and favorite customers. These are the people who we feel most comfortable with, who appreciate us, and who we spend most of our time working for (manufacturer) or selling to (customer). What we don't often do is check to make sure that the time spent is worth it - i.e. a good ROI.