in which scenario we should prefer visua studio 2013 over SharePoint desinger 2013

SharePoint Designer is a powerful tool from which you can deal with SharePoint at site level, create various pages, SharePoint workflows, libraries, lists etc. You can do mostly solutions in limited level like you cannot write event handlers and so on. Also everything you write is pre-compiled already.

Visual Studio is a development tool where you can develop various types of apps, sites, dynamic libraries, and it has rich level of functionalities to code, you can write event handlers and so many. You can choose to write code in various languags like C#, javascript, html, css etc. It also provides rich features for code management and debugging. You have to compile and deploy all custom features in SharePoint site which you will create by VS.

Hope this helps.