Justice Brandeis praised states as "laboratories of democracy", considering federalism is a way for states to experiment with different programs and arrangements before we try them on the national level. Think of how "Romneycare" in Massachusetts served as a test for Obamacare. Liberals are reconsidering their belief in federalism as they oppose the Trump administration--it's great for California to lead the way on climate change.
I hadn't considered until I read this post at Jstor how the U.S. itself served as a laboratory for democracy, an example for Canada of what not to do as they constructed their government in 1867. Notably, they wanted to avoid the features of federalism which had cost their neighbor to the south over 600,000 dead. They distrusted the 10th Amendment and the strong president (the dictator Lincoln).