Here we have the options, to set the processor affinity mask for all process, and also the I/O affinity mask. The default is to let SQL handle this automatically as above, as it will provide better performance for most workloads.
On a basic level, these settings restrict which CPUs can be used when restricting SQL server processing and/or SQL IO operations.
There are very specific use cases in which setting CPU affinity makes sense. Rod does a really good job of showing just why mucking with CPU affinity is not for the faint of heart.