By downloading the packages locally first, you can then safely disconnect from the internet before you install Visual Studio. The purpose of this feature is to frontload the downloading of the Visual Studio packages onto the same computer that you plan on eventually installing Visual Studio on.

To download a local installation package, select the Download all, then install option in the dropdown at the bottom of the Workloads tab of the Visual Studio Installer. You can use the Download all, then install feature from the Visual Studio Installer to download an installation package on the local machine before you install it locally, or you can use the command line to create a local installation package to install locally later. For situations like these, we've made other methods available for acquiring Visual Studio. For example, you might have an unreliable internet connection or your internet connection may have low bandwidth. Use the 'Download all, then install' feature

If you are an enterprise IT administrator who wants to perform a deployment of Visual Studio to a network of client workstations, or if you need to create an installation package of files to transfer to or install onto another machine, refer to our Visual Studio Administrators Guide and the Create a network-based installation of Visual Studio documentation.

Visual studio 2019 offline installer