Top Menu bar driving a sidebar sub menu

I have an application written is C# ASP.NET MVC 4 with Razor views.

Why use Razor Syntax?

Was doing some reading today about Razor Syntax with MVC Framework and was wondering why would/should I use Razor? What benefit does it provide over doing the same thing in the code behind and/or controller?

add web api endpoint, which return html from view

I have ASP.NET 5 Web Api and need to do the following task: