It lets you add details to objects that already exist in OSM. It looks very polished, has nice map, icons, and this default style of sticking to your actual location like some location based games for crazy people (I'm a former Ingress player, Enlightened FTW).
My neighborhood has all details about building size, roofs, street lights, you know if there are markings on the ground on street crossings (for people with poor vision), each path has clear info of what's it's made of, and stores has open/close hours provided. Super cool, easy to use and has real impact.
[0] https://streetcomplete.app/ [1] https://f-droid.org/packages/de.westnordost.streetcomplete/ [2] https://play.google.com/store/search?q=street%20complete&c=a...