There is a certain scenario for which having a dedicated button is needed. For video, especially, it sounds like a bad idea to dismiss the Lightbox on clicking somewhere in the overlay background. Because that may happen accidentally, and buffering the same video which just finished loading is terribly painful. This will not be the case for an image, assuming it is a decently sized image.

A couple of other reasons in the favour of having a dedicated close button are -

1. It is more concrete and conveys the function appropriately. Even though the button may not actually function, having it there somewhere in the corner helps convey the message properly.
2. If page loading it interrupted or slow, buttons can come to rescue for the user because they are visible.

There is a certain scenario for which a dedicated button is needed. For video, especially, it sounds like a bad idea to dismiss the Lightbox on clicking somewhere on the overlay background. Because that may happen accidentally, and buffering the same video which just finished loading is terribly painful. This will not be the case for an image, assuming it is a decently sized image.

A couple of other reasons in favour of having a dedicated close button are -

1. It is more concrete and conveys the function appropriately. Even though the button may not actually function, having it there somewhere in the corner helps convey the message properly.
2. If page loading is interrupted or slow, buttons can come to rescue for the user because they are visible.