(This is one of a series of Software Engineering Maxims Which May or May Not Be True, developed over the last few years of working at Google. Your mileage may vary. Use only as directed. Past performance is not a predictor of future results. Etc.)
It is possible for a team to work entirely from a task list, where Product Management and the Leadership add stuff and the team marks things off as they are completed. This is not a great way to work, but it is possible.
It is better if the team spend some fraction of their time on tasks which germinated from within the team - not merely 20% time, a portion of regular work should be on tasks which the team itself came up with.
- The team tends to generate immediately practical ideas, things which build upon the product as it exists today and provide useful extensions.
- It is good for morale.
- It is good for careers. Showing initiative and technical leadership is good for a software engineer.