Fitbit Product Experts Alumni are retired members of the Fitbit Product Expert Program. Learn more
I wouldn't really say it's replacing it as both have been available along side each other for many years. What is actually happening is that the Web dashboard is closing down.
As @Guy_ mentions, there are things that could be achieved through the Web dashboard that is simply not available through the app. One particular thing I like that I can't see in the app is lifetime stats.