When you first created your account and signed in, it's likely that you didn't enter all of your working maxes - either due to a time constraint or simply not knowing every single one of your PR's.
Because much of our programming is done with prescribed percentages, rather than outright weight recommendations, it will be useful for you to have as many Working Maxes as possible recorded in the database. The data that you record on this page will be used to auto-populate your workout weights. If you don't have a Working Max field filled out, that's ok too, but field will show a 0 for the prescribed weight. It will be up to you to choose a weight that makes sense for that exercise and set/rep scheme. Because of this, it's highly recommended that when you can, you update the working maxes page.
The good news is that it's easy to go back and update your working maxes.