So what is the reason for cutting off the bottoms and not just using the already there opening? (I hope this isn’t a dumb question lol)

DUH! Not a dumb question at all. I for got to explain WHY I do it that way: Less root disturbance.
Plant seeds. Grow seedlings. Wheeeeee..... Time to put them out in the garden. Dig the hole. Bring the baby plant in its pot over to the hole.
Gently peel off the lid of the container, ie, the bottom of the pot. Place the bottomless pot in the hole, gently wiggle the pot, and most of the time, it will loosen up enough to come off the dirt ball. Now slide it up over the dirt ball, and over the plant. Fill in the hole around the dirt ball. Done!
The taper (draft angle, if you like) of the pot means that it will easily clear the dirt and you won't have to try to get the dirt/plant out of the pot.