jbriden One use reason is to better control the executing node.
In my example I have a sequential stage where each stage in the sequence may execute on a different node. One of those stages that I would like to split into two stages, but can only do so if they execute in the same workspace because the second stage will depend on the local output of the first. If I could have a nested sequential stage, I could define the agent on the nested sequential stage each of its stages would then share that workspace.
My exiting alternative would be to stash the outputs form the first stage and unstash them on the 2nd, which (untested) seems like a lot more overhead and complexity than I justify for splitting the stage (especially if I were to split it into more than 2 stages).