Choosing
So which do you need?
Work backwards from the failure you are actually having, rather than from whichever service you saw recommended most recently.
Nobody knows the book existsPromotion — BookBub, Freebooksy.
Readers arrive and never come backList building — BookFunnel, StoryOrigin.
The book is not on the stores you wantDistribution — Draft2Digital, BookBaby.
Your ads spend money and return nothingTargeting first (Publisher Rocket), then the creative — and the creative is the part most authors never change.
Most authors need two or three of these at a time, and which two changes with what they are doing that quarter. Paying for a promo site before you have a mailing list, or for distribution before anyone knows the book exists, is a common and expensive ordering mistake.