Publishers edit books before the books are published. Even if you’ve spent a good amount of time editing your final manuscript before you submit it to the publisher, they’ll still edit it.
Your editor at a traditional publishing house also might ask you to make changes or write additional sections to make the book more complete. This is an important part of the publishing process because it helps ensure that you provide the best possible book for your audience.
However, it’s still critical that you’ve done your own content editing and submit your best work so that it impresses the publisher (and you get a book deal).