Have you ever wondered why your printer uses 2 black cartridges and only 1 of each colour. The reason is simple. In most Canon printers for example the large black contains a pigment based ink which produces very crisp text etc. The other smaller black contains a dye based ink which is used more for photo or image printing.By using the 2 black cartridges the printer is able to produce crisp, dark text and by switching in picture mode it can also then use the smaller dye based ink to produce the highest quality photos.Take a look at the video below as it explains this in more detail.
Another reason the printer manufacturers do this is to make extra money. By utilising an extra cartridge it's extra money in their pockets. Don't you just love printer manufacturers? You may also like to take a look at the differences between Dye and Pigment based inks.