system routes vs user defined routes

Although the use of system routes facilitates traffic automatically for your deployment, there are cases in which you want to control the routing of packets through a virtual applicance. You can do so by creating user defined routes that specify the next hop for packets flowing to a specific subnet to go to your virtual appliance instead, and enabling IP forwarding for the VM running as the virtual appliance.

Note: User Defined Routes.

For most environments you will only need the system routes already defined by azure. however, you may need to create a route table and add one or more routes in specific cases, such as: force tunneling to the internet via your on-premises network.

Use of virtual appliances in your azure environment.

In the scenarios above, you will have to create a route table and add user defined routes to it.