Sometimes, you want to take a photo of something, but the lighting is poor and you’re without a good tripod. Say, for example, your tripod is in the car, and you would miss an opportunity if you left to grab it. If your camera is small enough, you can prop it on almost anything and set a timer and you’re set. A tripod would be ideal, of course, but there’s no reason to miss a shot because you don’t have your tripod with you. Use whatever is available. I’ve used plates, books, shot glasses, my camera bag, and the list goes on. If you have a heavy lens on the front of your camera, you will need a spacer of some sort. Use your lens cap or a filter case as a spacer.
Again, the idea is not to replace the tripod. But in a pinch, there’s always something you can use.