The umpire will call Time before any interval, or if play is interrupted for any reason, or to announce the end of the day's play or the end of the match itself.
The call has more significance than just informing everyone on the field of play that play has ceased. A major reason for the call is that nothing can then happen that affects the match. For example, if the umpires were to leave the field to investigate a problem without Time having been called, the bowler could bowl the ball if he saw the striker was not ready without anyone being around to see that justice was done.
Although it may already have done so - perhaps because a wicket has just fallen - the ball becomes dead at the call of Time, and no further appeals can be permitted.