Why would someone buy a DSLR when you can take a perfectly good shot with your phone? This basically comes down to quality, speed and versatility.
1. The quality you get even from a basic DSLR will be much better than a phone or compact camera. The main reason for this is the size of the sensor. In a DSLR this will be a lot bigger than in a camera phone or compact camera. This will give you better pictures in low light and better dynamic range (more detail in the light and dark areas).
2. The speed comes about for two reasons one is how long it takes to activate the camera, turning it one and any delay between you pressing the button and the picture being taken. Your also get a faster shutter speed which is good spotting the action and getting a sharp picture. We all know the frustration of using a compact camera grab that instance with a fast changing scene.
3. Finally the versatility comes about with being able to change the lenses, add a powerful flash or extra battery power. You can adapt your camera to capture tiny bugs or attach to telescopes and get creative versatility with more advance controls. With a camera phone your stuck with a fixed lens, low power flash (if your lucky) and limited controls. With a compact camera things will only improve a little with say a zoom lens and a little more control.
All this said you are unlikely to carry a DSLR around with you all the time. The best camera will always be the one you have with you at the time. For most of us this will be the camera phone so you can’t beat it on this!
However there are times when you know your going to want to take some good quality pictures and it is convenient to bring along a DSLR. Maybe a family get together such as a wedding, a holiday or a day trip. If you’re a professional each assignments is going to be one of these events. This is where the DSLR comes in.