Now that you've made your way through the levels, you're probably going back to find alternate paths and collect all the red rings. (blogs.sega.com)
This opens up alternate paths through levels depending on which upgrade you opted to take, but sadly the differences are quite small. (wolfsgamingblog.com)
There are also many hidden bonus rooms (another series requirement), as well as alternate paths. (nintendoworldreport.com)