Digital BW, The Print

Today I've made my first prints with UT2 inks, with my Epson Stylus 870, and
I'm impressed by the tonal range and quality of the printed images.
Anyway, there is a little thing I don't understand. I loaded my 870 with an
eboni black cartridge (T007), and with the equivalent UT2 color cartridge
(T008). Next, I've used Paul Roark's curves, but every profile only uses
the color cartridge, even the xyz.eboni.acv ones.
So, the question is: what is the purpose of the black cartridge? After ten
prints it's still full, while the color one is at half of its capacity.

Best regards,
Claudio Valerio