What are the SVG pins for on the SainSmart Uno clone?
SVG = Signal, Voltage, Ground.

The Signal pin will carry the actual output, which may be high or low at any given time. It's basically just a male version of the corresponding standard GPIO pin. The Voltage pin will always be high (which can be 5v or 3.3v on this board, depending on the output level switch). The Ground pin is exactly what the name suggests -- it's connected to the board's ground.

This obviously means there is a lot of duplication. Why have two signal pins? And why bother having so many Voltage and Ground pins if they're all the same anyway?

The reason is simply convenience.

If you have a standalone external component (such as a motor), you would normally have to run 3 separate wires to it. With the regularly-spaced SVG pins on the board, you can instead use a 3-way cable with a suitable female header block. You can run that to an equivalent set of 3 pins on the external component. That means you are able to attach/detach the component by plugging/unplugging (effectively) a single cable.

I don't think the SVG idea is a 'standard' as such, largely because (in this form) it's only really of benefit to people who are experimenting or prototyping (as opposed to designing a more permanent system). It seems to be fairly popular among hobbyists and enthusiasts though. You just need to watch out for components which may put their equivalent SVG pins in a different order.