1. I don't have enough hands. It's different for each runner, but there are some things I really like to carry with me that just aren't pocket-friendly, like keys, cell phone, sunglasses...toilet paper. It's nice to have it all in a pack in case I need it.
2. I never know when nature will call. Since I mentioned toilet paper, yes, I like to take some with me. I've had bad experience with using nature's resources. That being said, I have discovered some good plants whose leaves work just fine for me (maybe in another post). However, I like the security of knowing it is there. (tip: keep some in a zip-lock bag to keep dry)
3. I never know what the weather is going to do. Certain times of the year, rain comes at a moment's notice. It is nice to have a few things prepared just in case. The pack itself can serve to protect a few things from the rain (my pack isn't completely water proof. So, as mentioned before, I bring a zip-lock bag to put in the toilet paper, maps, or other paper products. I also put in my cell phone if it begins to rain)
You could also keep a poncho in your pack, but I have never found it to be handy. First of all, it makes me sweat so much that I might as well be drenched in the rain. Secondly, it flaps in the breeze, and even though I'm not worried about it slowing down my already sloth-like pace, I find it to be very annoying.
4. I never know when hunger or hypoglycemia will strike. I absolutely hate having a pleasant run interrupted by my body screaming at me for food. I try to have two things handy: something straight sugar (jelly beans, hard candy...) and something that metabolizes slower with a little protein and even fat (like peanut butter crackers or granola bars). Another idea is to have some sports drink powder you can add to your water. (By the way, you definitely need to carry water with you. Many packs come with a water bottle holder. That is the one thing I actually don't mind having in my hand. I've run complete ultra marathons, including a hundred miler, with one or two bottles in my hand).
5. I never know when I will start getting a little chaff. I don't always have time to lube up, and if I don't need to I would rather just not do it at all. But it is silly not to have some anti-chaffing cream handy, because chaffing can sure ruin your day. When you feel friction somewhere and you can tell it is "heating up," you need to have some Body Glide, or Vaseline, or something to rub on the troubled spot.
The list can go on and on. I can't tell you what to pack or where to pack it--you will have to figure out what works best for you--but I highly recommend you always have some sort of pack ready. Keep it in your car or somewhere you can easily access it when that opportunity comes for another good running adventure. As soon as you can after a run, make sure you restock when needed.