Fortunately or unfortunately, it doesn't have anything to do with the available (or not) slider.
It's mostly helpful, at least for me, during days when I'm occupied with some other work (on Steemit or elsewhere) and my preferred author's posts would get lost deep down in my feed among other posts of all the members that I follow, especially if and when my preferred authors are in entirely different time zone.
As well, it helps a lot during those days when I'm not on Steemit, as my VP wouldn't just sit at its 100% power not being used what would be an entire waste of power as for those who might be rewarded with it, same for me as a curator, and above all for the whole Steemit community (as only through our activity the Reward Pool is getting filled).
The only important thing with auto voters is to pick the members who would be put there wisely. Meaning, to be sure they are not going to abuse the system by posting 5 or even 10 posts per day, create some BS and/or worthless articles, etc.