This diagram shows the different parts of a cam profile. In a valve train with solid lifters, the purpose of the ramp is to gently take up the slack before the valve starts to open, and to gently move the valve back onto its seat when closing.
Looking at this diagram you can see how roller diameter changes duration. The line denoting pressure angle is drawn from the center of the roller to the point of contact between the roller and cam. If the follower was a point, the follower motion would be the same as the cam surface. As the roller is made larger, it is engaged by the cam profile sooner as it opens, and later as it closes.