The primary reason for a "docking station" is for "docking".. Which means, when you have a whole buncha peripherals connected to your unit, whenever you go somewhere and come back, you don't want to connect all the stuff again to the laptop.. Monitor, serial cable, parallel cable, USB, keyboard, etc. etc. etc.
For those ppl that use a laptop as their sole computer, they can just come home and "plug it in" to the docking station, and then it feels like a normal desktop computer, without the hassle of connecting everything to the laptop.