When Object Enters Slide vs When Timeline Starts on

So it seems I'm on a knowledge quest ATM. I never really understood the purpose of the trigger Enter Frame, I get what it does, I just have never figured out why one would choose that trigger over When Timeline starts on.

I've been chatting with Google's AI and it really could not give me a good reason to choose one over the other. It argued that you could use a motion path and then as the object enters the slide the trigger happens. However, since it reads the very first bunch of pixels as the activator, I could also start the object at the very outside edge of the frame so while this means it will trigger before the object is ever seen, we are talking like 2 or 3 pixels difference.

I have used When Object leaves frame, BTW, I just can't recall a single time I've ever used Enter. Wise me up folks.