As I understand it,.on a flat tappet cam and lifters the there is a very slight angle ground in the lobes that ” forces” the cam to stay back in the block. The forces from the lifters sort of push it rearward. On a roller cam the angle is not there, it would cause issues with how the rollers ” track” on the cam lobe. That is when a cam button comes in. They are spring loaded and keep pressure on the cam, ” pushing” it back in the block and keeping it in the correct position.
Or something like that........
