There is a great site at http://www.ni.com/visa/why.htm that discusses in detail why programming with VISA would be beneficial.
In short, though, using VISA would allow you to communicate with your instruments regardless of the physical median: GPIB, Serial, VXI, PXI, etc. VISA is an industry standard with the goal of making software interoperable, reusable, and able to stand the test of time.
