They provide the cord in case you don't have enough molex connectors spare, which (your system being a Dell) you might not.
As stated above, the higher-end cards need their own little plug in addition to the power they get through the AGP port.
If you have no unused power connectors (they're all plugged into Hard disks & Cdroms etc.), then you can unplug a drive, insert the 'Y' splitter cable on, and then you get 2 plugs out of one, if you see what I mean.
At least I think that's what you're talking about, anyway
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"Sex without love is an empty experience...
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