Tasks and to-dos will expand to absorb all available time.
It's even more rare that doing everything that comes up is wise.
Objectives and key results are a way to focus on achieving specific results. They help you prioritize and think in terms of the outcomes represented by the objective/key result.
To help prioritize things, organize action items and projects in the context of the key result they support. Then, you can work on the things that provide the best set of outcomes in terms of reward, cost and risk.