Why do I need the NuGet Installer task if there is an option
on the Visual Studio Build task to Restore NuGet Packages?

The Restore NuGet Packages option of the Visual Studio Build
task currently only searches NuGet.org for packages. If you are using a service
like Visual
Studio Team Service Package Management those feeds are not searched.

By adding a NuGet.config file to
your solution where the other feeds are defined using the NuGet Installer task
will be able to restore packages from all your feeds. If you have already configured
your package sources in Visual Studio. You can simply copy your
NuGet.config file from %AppData%\NuGet and add it to your source control and
use with the NuGet Installer task.