why not? plenty of reasons

The other name of docking is "port replicator", which is exactly what is useful: when docked, there is power, mouse, keyboard, display connected, at least 4 cables. Depending on the work, e.g., my laptop have additional 2 u***** cables, 1 serial cable. Can you imagine plugging and unplugging all those wires when moving your laptop for a meeting? That's exactly why you need a dock.
Also, when docked, you can use both laptop screen and external monitor, so you have 2 screens. Super useful when doing lots of things.
Also, for Dell models, the dvi port is only available when it is docked. No dvi port on laptop itself.