Taking a series of shots in rapid succession can help increase your chance of capturing that critical moment in an action picture. Also, in sports photography such continuous sequences can tell their own stories that lead to the understanding of the movement and show the intricacies of a particular sport in a way that is rather not possible with the human eye.
In case of film photography, the number of frames per second that can be shot automatically dependeds on the capabilities of your camera’s motor wind. The maximum shooting speed of a digital camera relies more on processing power than on precision engineering. A digital camera is a computer, and the information recorded by the sensor needs to be digitized, processed, and stored—and this can lead to a noticeable wait before you can take your next shot. The card has to be at pace with the capability of the camera. If the camera is capable of writing faster, the card should also be such that it can be written on faster. Higher the resolution used, longer is the delay. Also, if your shutter speed is slow, the faster burst rate can only happen after the elapse of the longer duration of the slow shutter speed. So even if you are on a burst mode, the camera would not really shoot faster. ( The Burst would be as fast but will be limited by the slow shutter speed duration )
All cameras with a continuous shooting mode for action photography involve a buffer memory—a temporary storage that holds the unprocessed data as the sequence is being shot. ( just like a computer's RAM) The number of shots that can be taken per second at this “burst rate,” and the total number of shots that can be taken before the camera needs to stop and catch up, varies from one camera to another and this is one of the features you are paying for when buying an expensive camera. The file format and file size are also important—the rate will be sustained longer if you shoot in JPEG format, rather than in RAW. With a good SLR, expect a burst rate of 6 to 10 frames per second or more.
As per the specifications of Canon EOS 1D mark IV it can Continuously Shoot Max. Approx. 10fps. (speed maintained for up to 121 images (JPEG) or 28 images (RAW)) (with UDMA card)
EOS 1 Dx has a burst speed of 12 FPS but limited to 10 fps when the battery charge is less than 50% or when ISO speed is above 32000. If the camera's internal temperature is low and ISO speed is above 20000, the maximum continuous shooting speed is restricted to 10 fps.
EOS 5d mark III has a burst speed of only 6 FPS and the earlier mark II had only 3.9 fps.
Timing your shot is very important. Unfortunately, using a camera’s burst rate mode cannot guarantee catching the action at the right moment or at its peak. Even if you are shooting at 10 frames per second with a shutter speed of 1/100 sec, this amounts to capturing only a part of the action. Due to the limited capacity of the camera’s memory, and the maximum duration of the burst rate, you can not use the continuous shooting mode just blindly. Wait for the moment or when you are closest to that moment.
What is captured by the burst mode may not always show the right peak action which could be in an athletes jump or a dancers move.