1. Reduces the chances for errors, if a route is modified incorrectly in RouteConfig.cs then it may affect the entire application's routing.
2. decouple controller and action names from route entirely.
3. Easy to map two routes pointing to the same action.
Attribute Routing is a more flexible solution than Convention Routing, if only because it allows you quite a bit more flexibility and places the routes next to the actions that will actually use them. However there are certainly benefits to using both in tandem, particularly in situations when you know how some routes will look but aren't sure about others.