Most modern computer systems feature Multi-Channel Mode (commonly known as Dual-Channel, Triple-Channel or Quad-Channel mode), that require memory to be installed in a specific set configuration to enable the Multi-Channel Mode. The Multi-Channel Mode feature improves the transfer speed (or latency) of data between the physical RAM and the memory controller (located on the system's motherboard) by increasing the communication channels available between them. For example, Dual-Channel mode effectively doubles the amount of communication channels for data to move between the physical RAM and the memory controller.
Memory usually receives instructions 1 row after the other. Multi-Channel Mode as the name implies, splits the instructions and sends them down in multiple rows at the same time. This is why you need "matched set" of memory so that it is an even split. The reason most people don't see any improvement in speed is because the programs that they use are not supporting this feature (e.g. your email program just sits there until you starting typing a new email - most of the time your computer is waiting for you!).
For most everyday users that use their systems for tasks such as email, internet, word processing, etc. will not benefit from operating memory in these configurations as these tasks do not "stress" the memory subsystem that would take advantage of these mode.
For computer users that like to keep a lot of programs open at the same time (e.g. web, emails, iTunes, Excel). This requires a lot of physical memory to hold all the programs while the machine is running to minimise "lag" when switching between the programs. That is why adding *more memory* is always a good thing.
Multi-Channel Mode is great for intensive number crunching like rendering special effects in movies or the back-end servers of banks that process 1000's of internet banking customers per hour. After all, Multi-Channel mode was originally developed as a server technology but has filtered down to the consumer level over the years.