I basically use a day bag every day, whether at home or on the road!!! Always have my "essentials" with me, and when traveling, add a camera to the mix too!
I spent part of this afternoon modifying one of my old Hedgren nylon purses to add a slip pocket on the back . . since I couldn't find one made the way I wanted, I did it myself! It looks like I did it myself, but at least it's on the back and only I will really know it's there - and it will function like I want it to.
Edit: And what do I have in my purse + day bag (usually a small Hedgren backpack, sometimes a small canvas tote I made)? wallet, phone, chapstick, keys, metro pass, small handheld recharger for my phone, small foldable nylon shopping bag, hand fan if it's hot (not motorized, just the kind you fan yourself with), bandana if it's hot, small water bottle, small bottle of sunscreen, pens, sunglasses. For longer days out or when I'm really touristing: sometimes a baseball cap, sometimes a crushed up windbreaker, book or Kindle and/or magazine, small not-quite SLR (Canon G15), plus replacement battery, a pack of nuts and a chocolate bar, sometimes an umbrella, a small guidebook or map or notes/cards about what I'm seeing that day (or photocopies of my relevant guidebook pages), any necessary tickets, and sometimes a scarf.
And do I use what I carry? Sometimes -- some of those things are dependent on the weather. But I'm glad to have them if I need them, and they are small/light enough that I'm willing to carry them in case I need them. And that way I can function the whole day long without having to go back to the hotel for something.