By performing this reduction, Descartes single-handedly looked straight through the millennium of magical thinking and peeked into the Plato's Cave (senses can be deceiving). He rejected the notion that we have a complete free will and made clear connections between our sensations and bodily reactions, noticing the fact that we might not have any control over them (like reflexes). His mind allowed him to interpret mechanical fountains that he saw in the gardens he visited as bodily systems, leading him to more and less crazy but revolutionary theories (for more, look up: animal spirits or his view on the pineal gland).
The unsolved problem of dualism is still to this day one of the most intriguing. A discussion between fractions of dualists and monists today is as fierce as ever, and it does not look like there is an end in sight.
Other ideas, like the one with an evil, ever-deceiving demon, became a thing of their own leading to multiple philosophical views but also awesome films like The Matrix. There is no NEO without René, that is for sure.