None of the animation software I'm familiar with has "just a timeline". Just a timeline works for simple animations, but as soon as you have situations where you want the same animation at different points in time, it is very useful to be able to build up a library of small animations that you can apply at different times.
Blender's mechanism for doing this is through the action editor, and NLA; both of which use the timeline. The timeline serves three purposes:
- It is used to "scrub" the animation over time
- It serves as the focal point for adding keyframes to simple animations.
- It ties actions together in time.
The Dope Sheet serves two purposes, which overlap the timeline but are different.
It gives an overview of the actions in an animation, and is a carry over from pre-computer animation.
It is a container for the action editor, which could be a separate editor but isn't.
The Action Editor is used to create actions that can be reused, like walk cycles, or a character's slow smile emerging.
The NLA is used to put those actions together into a time sequence.
The graph editor, like the time line and the Dope Sheet, serves multiple purposes. In addition to allowing you to edit the curves used by the animation system for interpolation between poses in a pose-to-pose animation; it allows you to edit the curves used by drivers.
I would assume that the timeline is a separate editor rather than a subset of the Dopesheet for historical reasons; but that it remains separate because it is a convenient way of providing simple animation tools without having to introduce the complexity of action editing, or the information overload possible with a Dope sheet.