Why do you need to use a tent on a multi-day expedition?

With the advances of weather predictions that we have today we can pretty much dictate what equipment we will need for a camping trip. However, I have been camping and backpacking my whole life and I can tell you that it is still a guess. An educated guess but still a guess. The weather can turn on you faster than you could imagine. If you are going to be out for more than an overnight trip then you have to prepare for all possibilities. I have been on day trips that have become overnight trips due to the weather. Nothing can ruin a trip quicker than getting wet or worse getting snow bound. Shelter is the number one important thing that you have to be comcerned with. If you get wet then hypothermia can set in even in the summer. Nights can be cold or cooler than the days and if the temperature outside is less than your body temperature then you can get hypothermia. It can kill you. You may also need to escape from the bugs or the sun or the wind. Like the Boy Scouts motto says, "Be Prepared."