Why does .NET Core Web API calls AddMVC() and UseMVC()?

app.UseMvc() tells your app to add MVC to the request execution pipeline. This will ensure that all requests to your web application are routable to the MVC framework, meaning you can use controllers, views and anything else contained within the MVC implementation (action filters etc).

if you don't need view functionality then you

Don't create a web API controller by deriving from the Controller class. Controller derives from ControllerBase and adds support for views, so it's for handling web pages, not web API requests. There's an exception to this rule: if you plan to use the same controller for both views and APIs, derive it from Controller. The ControllerBase class provides many properties and methods that are useful for handling HTTP requests.

For more details checkout this link. https://docs.microsoft.com/en-us/aspnet/core/web-api/?view=aspnetcore-2.2

If we check the source code for the internal AddMvc extension we can see these things clearly:

public static IMvcBuilder AddMvc(this IServiceCollection services)
    {
        if (services == null)
        {
            throw new ArgumentNullException(nameof(services));
        }

        var builder = services.AddMvcCore();

        builder.AddApiExplorer();
        builder.AddAuthorization();

        AddDefaultFrameworkParts(builder.PartManager);

        // Order added affects options setup order

        // Default framework order
        builder.AddFormatterMappings();
        builder.AddViews();
        builder.AddRazorViewEngine();
        builder.AddRazorPages();
        builder.AddCacheTagHelper();

        // +1 order
        builder.AddDataAnnotations(); // +1 order

        // +10 order
        builder.AddJsonFormatters();

        builder.AddCors();

        return new MvcBuilder(builder.Services, builder.PartManager);
    }

In my opinion, for Web API, you probably need AddJSONFormatters() and AddCors(). However, you would also need AddMvcCore(). This includes stuff like routing, attributes, filters, result executors, model binders, controllers etc.

Refer to https://codingblast.com/using-web-api-asp-net-core-without-mvc-specific-stuff/