Part of the value in knowing yourself is it can enable you to have peace and joy. For me, there is a requisite spiritual aspect that must exist for attainment of either.
I will share my definition and perspective of each. Peace is a natural tranquility born from having a relationship with God while being able to accept where / who / what you are not. Peace is typically linked to older people as it often is associated with reconciling the life you lived (not necessarily the life you wanted).
As a younger adult, and even an older adult, I typically felt "settled" or "comfortable" in that I had a successful career, good to great relationship with my wife and children and a healthy relationship with family and friends. But my spiritual maturity and depth of personal knowledge precluded true peace until I was over 40. The combination of spiritual maturity and self-knowledge were critical enablers for me to truly obtain peace.
Attainment of self-knowledge included me reflecting on the decisions I made in life that put me where I am. It is through God's grace and mercy that I was in a good place up until I turned 40. With that said, we all are responsible for our choices, so the path I traveled was one I ultimately chose to be on ... personally and professionally; for there is only one path in life we can be on. So being at peace means accepting (mentally, spiritually and emotionally) what was not on that path up to where I am now. A simple example - I am not rich (financially) because I chose military service, and I did not achieve the highest ranking I potentially could have because of a career field choice I made. Peace is I fully accept this reality and hold no ill will, second guess or blame someone / something for what I hypothetically could have (if I chose a different path).
Joy is akin to spiritual happiness. It is not dictated by external events (good or bad happenings), but by appreciating how God has enabled you to attain / achieve what you have. It is persistent and consistent and permeates your being in all things, every day. I am not sure we can have joy without peace. For if you do not accept where you are not, how can you truly have joy for where you actually are?
It has taken me years to reach such a place in my life, and part of attaining such a state of mind and soul is knowing myself, knowing my path and being in relation with God via the Holy Spirit.