There are basically three things the legislation does.
First, it puts into legislation what's in practice in the ministry. This is something earlier governments have done; it's not an unusual thing. It also solidifies the one-tier ministry approach where everyone is on the same footing, and it resolves the issue of federal, provincial, and territorial relations in that ministerial title.
Second, it creates three new positions. If at some point in the future a prime minister were to decide to make a change, there would be a little more flexibility in those three spots.
Third, it allows the ministers in question, when they become full ministers, to get the support and the services to make the delegations required for them to do what's in their mandate letter. We have other mechanisms when they're ministers of state, and it allows for some flexibility there.