Noggin is (now since March) a companion streaming service from Nick Jr (it was originally a preschool channel before Nickelodeon dormant the brand; check out Jay's Corner's article). It provides multiple learning activities, select episodes from Nick Jr, and even original.
Noggin is supposed to be a direct competitor to the infamous ABCMouse service. The big difference was that Noggin has Nick Jr. episodes. It's also an add-on channel to online video providers such as Amazon's Prime Video, Apple TV, and the Roku Channel. Most Noggin original shorts have started to appear on Paramount+. Their recent new shows, Kinderwood and Noggin Knows are currently exclusive to the service.
The question is why would they do this if Paramount+ is around?
ViacomCBS stated they wanted to keep Noggin because of its learning values and I can definitely see that (Keep in mind they've never referenced Noggin during its February or Q1 events). But the fact Noggin is also an add-on service to video providers just shows that Nickelodeon mostly used Noggin as a preschool supplement to NickHits (both services were $7.99) before it shut down the day before Paramount+'s launch.
To be honest, if ViaccomCBS really wants it to stick around, Noggin should be focusing more on its educational values (learning activities) and less on being a video streaming provider with Paramount+ having majority of the Nick Jr. shows.