Setting boundaries and using them to say no can:
Help you feel balanced in the world (or, to put it another way, help you respect your own self enough to have boundaries, to say no);
Compel your to define for yourself the terms of your work (and what success looks like), relationships, state of mind, and uses of your time (at least as much as you have agency to);
Carve out real space in your life for spending time with the companions, communities, and activities that you most value;
Help you re-orient to a relationship with yourself that acknowledges your worth is not determined by productivity (so many wise people have said this!!!), and thus, give yourself permission to just rest, opt out, and do less "productive" things;
Support your efforts to accept yourself (and your skills and interests) with less guilt about what others expect from you (especially when those expectations are unsustainable or toxic).
While this document isn't the be-all, end-all for you mapping out how to say no (you can check out my whole collection of resources and articles on that here), this document provides some valuable advice about setting some boundaries that can help you harmonize how work functions as a sustainable, meaningful part of your life.