Because the clock will not stop for fouls or goals (injuries, time-wasting or other disturbances), so to prevent a team from protecting a lead by running the clock down during these stoppages of play, "stoppage time" is added or announced shortly before the 45- and 90-minute mark in the first and second halves.
However, many stoppages in play are entirely natural (e.g. throw-ins, goal kicks). An allowance is to be made only when these delays are excessive.
According to FIFA, the announcement of the additional time does not indicate the exact amount of time left in the match. The time may be increased if the referee considers it appropriate but never reduced.