I don't have one for writing - may never be a big enough seller to justify it - but I do use a virtual assistant from time to time thorough my day job. This has tended to be for specific things like a mass mailing (stuffing envelopes takes time), investigating travel options for work trips, researching costs for specific items, and audio dictation (I use technical language for work, and it's much easier dictating for a human to transcribe). The virtual assistant service is an independent contractor, so I just pay for the time I use - ideal.
For me, the critical thing is thinking of where my "pain points" are, and whether it would benefit me to have someone help out - for example, I have a cleaner because I loathe housework. It costs me money, but the cleaning gets done and I don't have to do it, so it's totally worth it!
In the writing sphere I've heard big sellers like Rachel Abbot, Nalini Singh and Elle Casey on podcasts where they talk about their assistants (a mix of physically present and virtual assistants). The sorts of stuff they use them for are monitoring social media, setting up email auto-responders, collating and updating series bibles, booking promos, mailing out signed copies, that kind of thing. I think some (not necessarily those named authors) also ask their assistants to respond to emails. Kevin J Anderson also uses a typist to transcribe his dictation (rather than using one of the software programmes).
I think a few have tried to use assistants to run ads for them, but this hasn't always worked well.
(I should say, this doesn't mean I know anything about the process - I've just been listening to a lot of podcasts!!)
