All processes in a process column participate. All All processes in the process grid participate. For instance, all of a distributed matrix row can be found on a process row, etc. Of course, how independent these rows and columns actually are will depend upon the underlying machine. For instance, if the grid's processors are connected via ethernet, we can see that the only gain will be in ease of programming.
Speed is unlikely to increase, since if one processor is communicating, no others can.
If this is the case, process rows or columns will not be able to perform different distributed tasks at the same time. Fortunately, most modern supercomputer interconnection networks are at least as rich as a 2D grid, so that these additional levels of parallelism can be exploited.