ComponentUI class contains lots of methods that takes JComponent as a parameter,
for example:

public void paint(Graphics g, JComponent c);

The ui delegates cast the passed component to the actuall component,
following example is taken from BasicButtonUI:

    public void paint(Graphics g, JComponent c)
    {
        AbstractButton b = (AbstractButton) c;
     ... }

It makes sense to generify the component type to take advantage
of the generic type feature