The addition we see in the Germans' form of Idealism is their response as faithful Christians to the undeniable success of Empiricism during the Enlightenment period. German Idealism provided modern Christians with a narrative to protect their faith while also accepting the observations that came from a camp of thinkers who were deeply skeptical of religious narratives. It allowed Idealists to interface with the observations of science while simultaneously marginalizing those findings as mere "phenomena" which cannot speak to the "true nature" of reality. Platonism didn't provide them with the defenses they needed to reject materialism since science by the 18th-century had advanced significantly, so they had to adapt new ways of explaining how Jesus the Christ could still exist in a world of machines.
As philosophical questions about the "Mind" were enthusiastically answered by materialists who grounded the unique, subjective experiences of life into universal, biological processes, Transcendental Idealists subsumed both "Mind" and "Matter" beneath the single banner of "Spirit". By the late 18th-century, Idealism was no longer adequate to explain the difference between air and oxygen, to explain how lightning was a discharge of electricity, to explain the similarities between fossils in the ground and living creatures: only chemistry, physics, and biology were robust enough for that. Kant (among others) saw the need to justify how God can still operate in a world of physical interactions. Like Gassendi with Epicurus, Kant stitched one philosophical world together with another, but, in this case, backwards.
If we turn out attention from Europe to America, we observe a similar trend in Ralph Waldo Emerson, who eventually rejected the orthodox philosophy of Harvard's school of divinity and incorporated Vedantic philosophy to help explain his position: both the mind, full of religious sympathies, and matter, which constitutes the beautiful, beneficent world of nature, are facilitated by the Spirit. This can also explain why ancient Hindu philosophy was so well received by Emerson, Schopenhauer, and others: the primary focus of ancient Indian philosophers was to reconcile Dualism and Monism. They provided extensive arguments to explain how they interface with each other, and this is exactly the sort of philosophy needed during the modern debate between Descartes' Dualism and the Monism of both Idealists and Materialists.