Both creation (writing) and promotion (marketing) are essential for a writing career however, since most writers don't have a business background, they somehow feel that if they continue writing, the promotion will take care of itself. Unfortunately, it won't. Marketing is essential.

It's essential for every writer, unless you want to remain a generic writer, who accepts low rates and wants to stay on the lower rungs of a writing career.  

Writer's Digest has an interesting article "How to Build a Marketing Platform" which is essential reading for anyone who's either in the middle of, or who's contemplating a writing career. It covers the topic brilliantly.

Here's the thing: you MUST promote every day, even if you're booked solid for months ahead. If you don't, in a few weeks you'll be looking around wondering where all the people went. 

Sometimes, the constant promotion can feel as if you're the ringmaster (and clown) in a three-ring circus, but that's the way it is. You need to not only make your peace with the necessity of promotion, but get highly creative at it. If you do, you'll thrive. If you don't... you need to get a day job.