Power Connectors and what not

If the card has a power connector and you don't connect a power cord to it, then your computer will halt during boot up and proceed no further until you connect the power cords correctly.

Any power connectors need to have power cables, if your PSU doesn't have the appropriate cables then get a different PSU.

The motherboard slot is only capable of providing 75w, and each power cable can give an additional 75. The motherboard slot can't do more than 75w, but if the extra cables have to beat 75 they can, but not by much. That is why some of them have two of these cables (IE the amount they need to pull off the extra cables is more than about 100w).

If the computer were allowed to operate without the cables and the video card tried to take more than 75w (which it would do, during normal operations, if it has the cable connectors) then as soon as that amount was passed you would be looking at potential hardware damage to either the motherboard or the video card. This is why the computer won't allow booting if this problem is detected.