That is huge because it not only gets the word out there but can help with author ranking because pre-orders will be calculated in along with other sales. (nocturnal-lives.com)
That is only approximate, because the correlation between author rank and earnings is not exact. (advancedfictionwriting.com)
Interestingly, hybrid authors ranked highest in the $ 100,000 + annual income choice, and did considerably well in all other income levels compared to traditional authors. (midnightpublishingllc.com)