Purpose, meaning, spirituality, religion, motivation, goals, dreams, aspirations. If you really think about it are all fabrications. Man-made, and predominantly in your head, things not of the physical world. I would say that anything your mind can think up isn’t real. Go with me on this.
It’s a hard and easy question, depending on how deep you want to go.
Many people want to capture a moment, archive a memory. Some people want to express themselves or other peoples stories. Some want to show you something, be it a travesty or crime to a shiny object to entice you to buy it. For others capturing a photo is meditative or therapy. We all have our own reasons to capture a photo.