What purpose could baseline humans possibly serve in a society of digitized minds?

Population Growth As @c.z. touched on, digital minds are just that - digital minds. If you clone a mind, that's all it is - A clone of the original, or originals. It wouldn't be unique. Adding a biological component would make it truly unique.

Adding on this, as others have said the numerous costs involved in digitizing a mind make it so that not all minds can or should end up digitized. Only the best and brightest (And politically connected/rich) could meet the requirements to be digitized. This would also make humans a lot more "disposable" in the sense that losing a single human is a lot less of a loss than all of the effort, time, support systems, etc that go into a digital mind. This, combined with the human body's natural durability and self-repair/healing, make flesh and blood humans particularly suited to high-risk jobs - Which could very well be incentivized by being fast tracks to becoming a digital mind.

There's also going to be the groups of people that just don't want to move forward. There's always conspiracy theorists, religious groups (Like the Amish), political groups, and people who are just not interested in becoming digital. As the digital minds were still human at one point, it wouldn't be likely that they'd go on a campaign of extermination. Just as long as nobody interfered, then they'd be ignored.