Suspension Geometry question

What is it about putting a lift on the suspension causes the need for new UCA's? As I understand it the lift (well coil overs) fits between the two Control Arms right?

 

It replaces the stock upper ball joint with a uniball.

 

The coil over mounts between the lower control arm and the coil bucket on the frame, the alignment adjusters are on the lower control arms,, when you move the lower control arm down (as in lift the truck) it can become difficult to achieve enough castor to get the alignment into spec.
After market upper control arms are usually built with more castor to allow alignment.