See here, the trigger editor controls many of the aspects of the game. The trigger editor allows you to add different things like gold, lumber, resources, create units at specific points, and many things. The Object Editor is very limited, and that's one of the reason people use triggers instead.
You can also deal multiple damages consecutively, configuring the damage and attack type created, which most of the times the Object Editor can't.
I can go on and on about why the Trigger Editor is more free that the Object Editor, but I'm not going to be able to fit that into one post.
Also, the Object Editor cannot assign damage to 'specific units' but to units in general, like heroes or machines, not specifically like pre-placed units.
Let's see you or anyone here create the spells in the Spells section using only the Object Editor. That's impossible. That's just impossible to do.