The advantage is if the chipset supports dual channel / is a dual channel platform.
From Crucial FAQ:
What is dual-channel memory?
The terminology "dual-channel memory" is being misused by some in the
memory industry, which can mislead the consumer. The fact is there's
no such thing as dual-channel memory. There are, however, dual-channel
When properly used, the term "dual channel" refers to the DDR or DDR2
chipset on certain motherboards designed with two memory channels
instead of one. The two channels handle memory-processing more
efficiently by utilizing the theoretical bandwidth of the two modules,
thus reducing system latencies, the timing delays that inherently
occur with one memory module. For example, one controller reads and
writes data while the second controller prepares for the next access,
hence, eliminating the reset and setup delays that occur before one
memory module can begin the read/write process all over again. Think
of it like two relay runners. The first runner runs one leg while the
second runner sets up and prepares to receive the baton smoothly and
carry on the task at hand without delay. While performance gains from
dual-channel chipsets aren't huge, they can increase bandwidth by as
much as 10 percent. To those seeking to push the performance envelope,
that 10 percent can be very important.
If you have a dual-channel platform and you want to take advantage of
the performance gain it offers, our advice is to simply purchase your
DDR or DDR2 memory in pairs. However, be very careful to order two
modules with the exact same specifications; the modules must be
identical to each other to perform correctly.