I suppose if you to rewrite the sentence as

Perry White could yodel, but this is not why he became the editor of the daily Plannet
would then give you an independant clause... He became the editor of the DP

(BUT THIS IS NOT THE BEST EXAMPLE< I AM JUST TRYING TO REWORK THE SENTENCE... it is not really the best example, becaues the phrase is intended in its use to emphasise Perry's other talents...

So a better example would be:
PW was the best EIC the Plannet had ever had because he knew the comings and goings of all his reporters, keeping up to date with every comma as it was written.

HERE this is an independant clause as Perry's knowledge of his reporters would make sence as a sentence, but in tying the sentences together we gain more information as to the bigger picture

EDIT:
Whereas with: Keeping up on every comma as it was written:

Is a dependant clause that describes Perry as he is keeping an eye on his reporters

You can't have MANSLAUGHTER without LAUGHTER

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