You are configuring and securing a network environment.

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 appliance. 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.