The "developer day" concept (one day a week to work on whatever projects) serves several ends. First, a team should have about 30% slack time where they're not focused on retiring backlog items. This is basic Lean theory. 1/
Imagine what would happen if fire fighters were busy 100% of the time, with no slack. If a fire started at your house while they were busy elsewhere, you wouldn't be too happy. Teams without slack are less productive because you're adding unnecessary wait times, etc. 2/
No slack also indicates that you have too much work in progress, which also slows the team. Next, it's important that the team be constantly acquiring new skills so that they'll already be in place when you discover a need for them. 3/
Working on individual projects does that. Also, individual projects give people the opportunity to add "purpose" (in the Dan-Pink autonomy/mastery/purpose sense) to the week. 4/
There's no requirement that this work "benefit the business." in fact, that's contraindicated if the point is to acquire skills that will benefit the business a year from now, but not today. 5/
That said, I see nothing wrong with people spending the time working on business-related tasks that bug them, but which they wouldn't normally have the time to work on. 6/6
