Infractions are used to serve as a reminder and a point of contact regarding the forum rules.
They are typically used in the not so serious instances. Sometimes for repeat low level problems or for slightly more serious one of cases. In both of these cases PMs/time-out/bans might be sent but it depends on the history of the problem(s) however the Infraction system is used as a visible reminder regarding the rules.
Sometimes they have to be used in situations when a normal PM would be suffice as members might have PMs disabled and thus the only point of contact is an infraction.
That covers the broad general use of infractions and why members might be given them. If this does not cover what you wanted to ask please ask a more definite question(s)