Bug and work tracking
For most of their history, computers have excelled at doing things one at a time. Even a single client or customer can parallelize much better than that and will think of (and make) multiple requests while you're still working on one thing.
It's really useful to write all of these requests down, and keep track of where you and your colleagues are on each of them so that you don't all try to solve the same problem, and can let the client know which of them you've fixed. Bug trackers (sometimes more generally called issue trackers or work trackers) are designed to solve that problem.
What Goes in And When?
I've worked on projects where the bug tracker gets populated with all of the project's feature requests at the beginning (this discussion overlaps slightly with the treatment of software project management patterns, in Chapter 13, Teamwork). This introduces a couple of problems. One is that the Big List needs a lot of grooming and editing...