Education in the US is pretty decentralized but there's plenty of work the federal DoE does that's very important, here's a good place to start looking: https://en.wikipedia.org/wiki/Office_for_Civil_Rights
The DoE enforces many of these laws which prevent abuse, bullying (teacher tolerance of it), discrimination, etc in education, which is pretty important. It's almost certainly the case that the DoE has seriously impacted the education of you or someone you care about through lawsuits, intervention, investigation which all change policy at the local level in a big way.
There's plenty of more things that it does. Take some time to look at what the DoE does and answer the question for yourself, it isn't hard.
In particular there are a lot of states that would love to let private religious schools torture gay children to try to turn them straight. The DoE is the only reason they can't do that.
- regulating how states are accountable for standardized tests (No Child Left Behind, later Every Student Succeeds)
- regulating federal student loans
- civil rights (LGBT, race, etc.)
- federal assistance for low-income students and underperforming schools
They don't dictate curriculum or anything. So basically it falls under the old argument of Federal vs State control over education. Conservatives have been trying to get rid of the Dept of Ed ever since it was made a cabinet-level position in 1979. They might now have their chance.