You import your build (or even just a single part) and use their compatability filter to shop for other parts from a variety of suppliers. I’m not sure how accurate it is and there are some minor issues (had a liquid cooler not fit a motherboard but it wasn’t flagged on the site) but for the most part the compatability filter is accurate.
You can also use it to make your ridiculous dream builds just for fun, plan out other upgrades, shop for the best price, and keep track of what your current build/build history is.