Why flow of real power depends on the phase difference of voltage magnitudes?
Refer to "Power Flow on AC Transmission Lines" for a slideshow.

With apparent-, real- and reactive- powers defined as:  $$ S^2 =P^2 +Q^2.$$

The real power \$P\$ flow between two buses is obtained by:
$$ P = {V_1 \cdot V_2 \cdot \sin (\delta) \over X}$$
and the reactive power $$ Q = {V_1 \cdot (V_1 - V_2) \cdot \cos (\delta) \over X}.$$