I've found that most of the people I know have their own way of capturing the world around them. Every day I see people take photos, write, paint, sew, play music, etc.
I feel like these are ways to hold close the beautiful and interesting things around you. It turns the moments, the light, the texture of the world and makes it physical. Archive-able. Sharable. You could say that it's a legacy of things you think about and notice. The part of your personality and aesthetic you wish to pass along, or at least record.
Walking around with something that I can capture the world around me with helps too. I feel like it sometimes it makes me more observant. It's like a little voice in my bag saying, "Look at that! Look at this!" I look for the things that inspire me. On a bad day, having that reminder to look for things I admire or that fiddle with my brain helps to reaffirm my satisfaction with my life and the world.
And at other times, it's handy for wonderful (and I mean that most literally) things I happen upon purely by accident.
And sometimes it's good for being selfish. Capturing to have for always. This piece of the world and of time to hold and think about. Sometimes something has texture that makes me want to rub my eyeballs on it. Sometimes a moment feels like a sound, or a set of words. Sometimes I want to wrap myself in soft things, pretty things, turn myself into a shape. Sometimes I want to put things that have survived human history under a glass jar to ogle, to appreciate, to mock.
I personally do this for vanity and selfishness and hedonism. I like reading the things that I write (sometimes) and looking at the photos I take, because (if I'm successful) they evoke in me the emotions I had when I wrote or photographed. Maybe it's because I'm a control freak and I like setting up my own stimuli.
Others might want to send a message straight into the retina. Make pupils contract. Make sphincters tighten. Make hearts beat and neck hairs stand on end. Sometimes they want people to be outraged, to be sympathetic, to be sad, to be joyful. Essentially sometimes they want people to care about something. These are worthy causes, especially if there's something important to say with these snippets of the world.
I don't really do that yet. Gotta work my way up to it, i guess.
Rule: Capture something of the world at least once for yourself or others.