Experience Best Practices for Improved Engagement

Hello. I have a modal/Experiences question. If you have an Experience that is stand-alone - not in a goal, how would that Experience get prioritized for being shown? I am trying to troubleshoot why a modal is getting very low engagement and want to be sure I consider everything that might impact the serving of the modal.

What is the mechanism governing modal priority across all modals/segments?
Should Experiences always be part of a Goal?