The dotnet restore command uses NuGet to restore dependencies and project-specific tools specified in the project file. In most cases, you don't need to explicitly use the dotnet restore command, since a NuGet restore is run implicitly if necessary when you run the following commands:
In previous versions of .NET Core, you had to run the dotnet restore command to download dependencies immediately after you created a new project with the dotnet new command, as well as whenever you added a new dependency to your project.
In .NET Core 2.0, you don't have to run dotnet restore because it's run implicitly by all commands that require a restore to occur, such as dotnet new, dotnet build, dotnet run, dotnet test, dotnet publish, and dotnet pack.
To disable implicit restore, use the --no-restore option, as shown below.
dotnet build --no-restore
Building requires the project.assets.json file, which lists the dependencies of your application. The file is created when dotnet restore is executed. Without the assets file in place, the tooling can't resolve reference assemblies, which results in errors.
dotnet restore command is still useful in certain scenarios where explicitly restoring makes sense, such as continuous integration builds in Azure DevOps Services or in build systems that need to explicitly control when the restore occurs.