1) I used to write a lot on Quora. Being an engineer who can write made my future VC partners believe that I could do more than engineering. I'm pretty sure that building a decent readership base on Quora is what put me on my partners' radar in the first place.
2) Writing on my blog/Twitter/etc has helped put my name and my fund's name on more people's radars and generated a lot of leads for potential investments. As an introvert, writing has been an excellent solution to not wanting to go to networking events/dinners/etc. :)
On the startup side, writing/content marketing are great ways to get customers, especially for b2b companies. Having a strong product matters, but being able to articulate the product's value and your company's expertise through writing is incredibly helpful.
In my experience my ability here has always been valued after the fact; I have the strong impression that few companies value writing, even as a secondary skill, or see it as a differentiator.