Learn by Doing
For every conversion project, the DPS process provides a "school of experience" through which publishers can build accessible ebooks in a working environment.
The applied learning that comes from engagement in the process and the Q&A which inevitably accompanies every order is invaluable for editorial and production staff.
The support tickets generated with every order amount to a "textbook" customized to your organization's needs and experience.
Any questions that arise from experience with formats, difficulties with distributors, are expertly answered in the context of a particular book, which over time builds an applied knowledge base for your list and your books.
As outlined here, there is a bewildering variety of ebook and print-on-demand vendors and format choices.
Distributor Challenges
The same could be said of distribution options of which there are hundreds.
Every distributor offers to format your manuscript to suit their requirements.
Distributors like Kindle, Kobo, Apple and Google, etc
Aggregators and Self-publishing services like IngramSpark, Lulu, Smashwords, etc
Specialized platforms like Gale, VitalSource, etc
Every distributor provides instructions for rendering books in the formats each requires. But UA PDF and ePub3 are accessible formats universally accepted

See lists:
Note the "Third Party Conversion Services" section, which includes this:
"The preferred outputs from conversion houses to be processed by Amazon are:
- Books in EPUB format
- Metadata in ONIX format (XML)"
Troubleshooting
There are so many variables in the process, from ever-changing ePub standards to distributor practices. Doing ebook conversion inhouse means that you're paying for troubleshooting. With DPS, when an epub fails a test or a submission fails to meet a distributor standard, the troubleshooting is free, and the learning experience is permanent.