It means that you pay a price premium for nothing basically. SLI-ready MB of course has two suitable PCI-E slots so that matters.
The SLI Mem is no more expensive then its equal rather I should say its within 5 dollars.
The SLI memory will tell the MB what it is and you don't have to mess with any setting for the memory (on a MB with this as a feature) for instance on an EVGA 790i Ultra board and you put in 1800MHz SLI (OCZ) memory the BIOS reads the EPP 2.0 modules that are programmed to boot at 1800MHz and violia your memory is now set and running with the correct timings and volatges and at 1800MHz.
So it gives you a "no brainer" of setting your memory at a higher speed without having to tweak the BIOS. It will also allow you to obtain a higher FSB speed in relation to the memory speed without throwing voltage at the memory and only the NB. Making it easier to obtain or get closer to a 1 to 1 ratio at higher speeds.
This is also true on the Triple Channel XMP (Intel) for x58 boards
OCZ
NVIDIA SLI Memory 1800 4GB 2 x 2gb ~86.00
Reaper Memory 1800 4GB 2 x 2gb ~87.00
Platinum Memory 1800 4GB 2 x 2gb ~82.00
