Why is there a partial double bond character between the C-Cl bond in vinyl and aryl halides?

The lone pair of  electrons on the chlorine are in conjugation with the  $\pi$      electron cloud of the vinyl or aryl group.

Kindly refer to the following link:
https://www.meritnation.com/ask-answer/question/vinyl-and-aryl-halides-are-comparitively-less-reactive-than/haloalkanes-and-haloarenes/400260