Maya is based on a graph of connected nodes. You can see this most clearly by creating a cube and graphing it in the node editor.you will see a creation node which provides info on how the cube is made which passes that info to the shape node, which is the main node for your Geo. Certain operations like extruding or revolving nurbs will also have their own node, also passing info into the shape node. That is why they are listed under inputs in the channel box.

Now this is good because the flow of info is live and dynamic. For example, you can change the number of cube divisions in the creation node even after your cube is created. Or tweak a curve and the revolved surface will update accordingly. So having that history intact is great for making those afterwards edits.

But, you can see how this slows down calculations because maya has to pipe the info through many nodes after a while just to draw your shape. When you are finished with the input nodes, you can delete history and maya will bake all that info through the input nodes to your shape node / Geo. Now maya has fewer calculations to do and it takes less memory.

I hope that makes some sense. It takes a while to get familiar with how things work, but if you think about it all in terms of connected nodes, you will understand much more.