why isolated components

[Laszlo Varga]

Hello,
may I have a very basic question?

The component based systems has proofed the basic idea of decomposition.
In mechanics, you must have components to get something move.
Since the invention of the wheel everybody goes for components, but only some 
knows
why.
Really, why?
What are the (main) motivations for kind a system?

It might sound stupid, but I hope that the precise answer for this
would help the rest of thinking.

Cheers
Laszlo