The previous pages on lapply-based, foreach-based, and alternative forms of parallelism are all optimized for parallelizing problems where the CPU is the ultimate bottleneck between you and your results. As mentioned in the Caveats with lapply- and foreach-based parallelism section, though, there are some problems where the size of your dataset is the biggest hurdle because it cannot fit into the memory of your workstation.
The map-reduce paradigm works around that limitation by splitting your dataset up into manageable sizes before your parallel R scripts begin processing them.
Unfortunately I have not had the time to turn this section into prose. If you are interested in learning more about using Hadoop with R, please contact me and let me know! I have the material for this section in slides already; I just haven’t had the motivation to turn it into a tutorial.