Batching in MSBuild essentially exploits the feature of cl.exe to take an arbitrary number of input files with the same command line arguments (with an inherent way to map input file to output file one by one) and make a single invocation for that batch of items. This can considerably speed up builds and it relies on the client/server architecture built into cl.exe or other suitable tools to distribute the work to a pool of processes (process creation in Win32 is relatively expensive, so keeping processes around instead of spawning new ones makes a difference).
This also includes test projects that I use to dabble with MSBuild in order to retrieve circumstantial information during a build (e.g. the compiler version: ClVersionRetrieve.targets).