Physics entity and collider

I just watched the tutorial about entity and cilliders. I have a question about a pricipe I don’t get. So 2/3 objects in the video just get an entity and 1 of them gets a collider. Even to the block and cylinder don’t get a collider they still interact with eachother as shown at the end of the video. What is than the point of adding a collider?

However when you model your own physics conveyor and you only add a entity the physica object will fall through it untill you also ad a collider to the conveyor. Why do the objects interact with eachother in the video without a collider but a conveyor needs a collider to interact?