Hello,
So as you work in Maya and on particular objects lets say a polygon cube, you build up history/nodes on your model. So if you create a bevel you can edit these attributes of this bevel as you go along. Then if you cut into your model you would have a polySplit or a polySplitRing Node. As you create more edits you will be storing a lot of information in your scene about your model. So the more history you ave the more nodes are required.
However once you delete the history all these nodes go away and your stuck with the model as it is and you can't go back to previous nodes. So delete history when you're happy with your model. Deleting the history speeds up the computer/Maya because it has less to compute.
Also the difference between non-deformer history and history is that non-deformer history will keep certain things like skinned nodes and things that deform like bend and twist deformers.
Hope that cleared up some things.
