The rackmount on the 555 can be removed and reinstalled in the field easily. You can use one instrument for both benchtop and rackmount applications. Yes, because the need often exists to have synchronized timing control signals. Also typical is the need for multiple pulses at a node while other nodes have only one pulse. An example of such an application exists is fluid dynamic studies where cameras, shutter, flash lamps and Q-switches are all synchronized to each other.
Some pulses go to independent inputs.
The 555 arrives with a factory default setting programmed into the unit. You will get pulses out immediately upon turning on the instrument and pushing the RUN button. The human interface was well designed to shorten your start-up time. There are two types of menus - operating menus and channel menus. The 555 provides features common to all channels and features specific to a single channel.