It's a proprietary Dell connection. They've done this for years with a number of their video cards. You'll need a (you got it - proprietary) splitter that will likely give you two VGA outs or a DVI+VGA.
This is not proprietary. This is not Dell specific. This is not low profile card specific.
It is used for multiple monitors. Many Quadro and Matrox cards have it for multiple monitor support. I even have an ATI X2400 at my desk that has this connector.
We use these cards for our 12 headed (and 10 headed) beasts that we drive at work. Our help desk wall is composed of 2 PCs running 3 video cards each (22 monitors total). Each video card h(in my sistuation) has this connector because it can drive 4 monitors.