This allows you to focus on those items you value the most.
Decluttering utilizes your decision-making and problem-solving skills:
You've got Y amount of space and X amount of stuff.
To make everything fit, you need to make reasonably quick decisions about whether to keep or discard items, and where to put them.
Doing this successfully can help you feel confident about your decision-making skills. It helps you feel like you're adulting!