Digital BW, The Print

So I've got an Epson R1900, currently with OEM carts that I'm considering replacing with refillable carts. I've been reading the refilling instructions here:

http://www.inksupply.com/instructions/r260_slc_refill.pdf

And I keep getting hung up on one thing...why remove the cartridges to refill them? If you're refilling from the top, not removing them seems to avoid the need to re-prime the bottom port.

This especially makes sense with the InkSupply carts, since they can be reset just by pressing the ink change button once they hit empty.

I'm super new to this, so I'm sure I'm missing something critical. I'd appreciate any info the group could provide to help me along.

Thanks--
Greg