you can use a passkey solution (passkey w/real cart, passcard, datel max media launcher, etc.) in conjunction with a flash cart (G6, M3, supercard, etc.) to run homebrew, images, movie files and back-ups.
the passkey/passcard has code that forces the ds to look to the gba port to run the software from.
flashing your ds's bios with flashme, alters the existing factory installed firmware to eliminate the need of the passme/passcard portion of the above mentioned solutions and thus requiring only a flash cart in your gba slot to run things.
hope that clears things up for you.