Boundaries are a reminder for you, and a signal to others, that you are choosing to be deliberate about your plans and goals. When you do not have or honor your boundaries, you risk being pulled in random directions. It is much less likely then that you will make meaningful progress on the things that really matter to you.
Boundaries are a way to stand up for yourself and to stay available for own equally valid needs.
Standing up for yourself can help you feel like you have some control over your life because you get to determine how you will use your resources instead of letting everyone else decide for you.
Boundaries can help reduce stress and may even reduce outbursts of anger or frustration that can come with feeling put upon, overwhelmed or just plain invisible.
A final thought...saying yes to everyone except yourself provides a poor example to others for how you expect - and deserve - to be treated.