An independent author can upload directly to the largest retailers, yet many route their books through a distributor instead. The reason is that direct access stops well short of covering the market.
Most retail channels are not open to individuals
The biggest ebook stores accept uploads from anyone. Many others do not, particularly library systems, subscription services and retailers operating in single national markets.
Those channels contract with approved suppliers rather than with thousands of individual authors, because vetting and paying each one separately is not worth their administrative effort.
A distributor becomes that approved supplier on behalf of everyone using it, which is the whole basis of the arrangement.
One upload becomes many listings
Each store has its own file requirements, metadata fields, category systems and pricing rules. Managing them individually means repeating the same work in a dozen slightly different interfaces.
A distributor takes one file and one set of details, converts them, and pushes the result outward to every connected retailer.
Changing a price or correcting a description then happens once rather than a dozen times, which is what makes a large catalogue manageable for one person.
Payment and tax handling is a real service
Selling internationally means dealing with withholding rules, currency conversion and multiple payment thresholds, each store paying on its own schedule.
A distributor consolidates all of it into a single payment and a single statement, and handles the tax documentation that non-resident authors would otherwise complete for each retailer separately.
For an author selling small amounts through many stores, this consolidation is frequently worth more than the commission it costs.
The cost is a share of every sale
Distributors take a percentage of revenue before it reaches the author, applied on top of whatever the retailer already keeps.
On the largest stores, where direct upload is available, that share buys nothing the author could not have arranged alone.
The common approach is therefore to go direct to the major retailers and use a distributor for everything else, accepting the fee only where it purchases genuine access.
Control still rests with the author
Reputable distributors take no rights in the work and can be left at any time, with listings withdrawn on request.
The distinction worth checking is between a distributor and a publishing service that claims rights, since the two are sometimes marketed in similar language to writers new to the process.