(DSLR meaning digital single lens reflex camera and P&S meaning point and shoot - simple pocket camera)
There are many reasons:
- the ability to switch lenses, allowing
to have a much bigger range of possibilities in terms of focal length, aperture (speed, DOF),
optical
quality
- low noise or better noise performance thanks to larger sensors,
resulting in less noise in the picture
- better autofocus, improving speed and reactivity - the measurement
principle, TTL (through the lens) makes the AF inherently much more efficient on a
reflex camera. The autofocus on a DSLR camera is based on a separate specialized sensor allowing much faster and accurate reaction.
A P&S camera, on the other hand, is just analyzing the picture forming on the sensor, using an algorithm to detect local contrast
(sharpness) - this is a long and costly process resulting in a much slower reaction time
- speed (number of pictures/sec)
- reactivity in general including start-up time. The camera is virtually immediately ready to take a picture
- build quality and weight are an advantage for intensive use
- wide range of accessories (remote controls, flash systems, wireless
or not,
angle viewfinders, ...)