Distributed monitoring is a method of website monitoring when the checking is performed from several locations. The main purpose for this is to exclude errors of checking server (which is always possible) from the site statistic and provide more precise result. Usually it is realized through the network of independent servers which check the sites one-by-one or simultaneously. The advangtages of such checks are listed below:
Checks are happening from different locations, just like the real users do.
If a single check fails - others may prove or decline the failure. So, probability of false downtimes is really low.
It is possible to overview the access and download speed from different countries and cities.
Possibility to catch network-related or DNS problems: site might be visible from your computer, but are you sure it works for everyone?