AutoScrolling does not allow you to dynamically control the scrolling interval. If you are drawing a complex control such as grid then you want to be able to scroll based on the current row height, column width etc. With AutoScrolling you cannot do this. You have to implement Scrolling yourself in such cases.
Autoscrolling is also not very useful if you want multiple views to share a scrollbar. The most common place where you see this is with a workbook. There is no direct way in Winforms to hook up your own scrollbars with the AutoScrolling implementation.