I great video on the importance of trigger.
''An oscilloscope's trigger function is important to achieve clear signal characterization, as it synchronizes the horizontal sweep of the oscilloscope to the proper point of the signal.''
Sources:
Episode 7 - An Oscilloscope from 1946 Can Still Teach Some Lessons - Jack Ganssle :
https://www.youtube.com/watch?v=zkFddiu2Dvk
Second video: