In Spirituality, there is the idea of "I Am". But what does it mean? Well, it can mean many things to many people.
Biblically, Jesus refers to himself as "I Am" seven times. Many Christians have taken this as Him calling himself God eternal.
In terms of affirmations, it helps our mind to use I Am instead of I Want. Our subconscious mind doesn't work in terms of past, present, future. That's why many affirmations use I Am, and also why manifestation often uses terms in present tense. Using I Am conveys you already have what you want.
For existence, I Am can simply mean the moment when you feel your calling. What motivates you to feel the full energy of the universe. What fuels your existence. When you feel your "I Am" moment, your heart feels full and happy and you feel complete. Going back to a biblical perception of what I Am means, your I Am could also be when you feel most connected to the Divine.
Let me give you an example from my own life. I feel I Am when I work with my tarot and oracle cards. I feel I Am doing my crochet work. I feel I Am reading about the Divine, no matter what form. I feel I Am while on a nature walk. I feel I Am deep in meditation.
That's just me though. Your I Am can be big or small. It's personal though. To know I Am, to know that feeling, is life changing.
But those are just my thoughts.