One way uncommitted objectives help Agile Teams make a plan for the PI is that they maintain the predictability of achieving the objectives. Uncommitted objectives are PI objectives that are not included in the team's commitment or counted against teams in the ART predictability measure. They are used to identify work that can be variable within the scope of a PI, such as stretch goals, spikes, innovation, or exploration1 Uncommitted objectives help improve the predictability of delivering business value since they allow the team to adjust their scope based on the actual progress and feedback during the PI. They also provide the team with some flexibility and capacity to respond to changing needs and expectations, as well as to pursue learning and improvement opportunities12 (Must be taken from SAFe 6 Scrum Master resources) Reference:
PI Objectives - Scaled Agile Framework
PI Planning - Scaled Agile Framework