Why should I use web API

Question

I am going to start a campus management system its scope is a bit broader , right now I am not clear which specific design pattern and architecture pattern should I use , Should I use normal Controller , Model and views approach as in normal mvc templates or should I used three tier architecture along side with the default MVC architecture.

 

WHY SHOULD I USE WEB API? 
I knew web api return data in generic format XML and JSON , and multiple clients can be handled with it , but right now I do not have a plan to have mobile application as well for this application .So i just would have web application , 

 

the MAJORS which i need to consider right now is RUSH development i have short time to do that entire stuff.

 

can someone share his thoughts alongside with any refernece project which is available here to see .
My Second question I have copied the frontend admin template from some where , but it is in html not in razor view? is it necessary to have in razor view OR is there any shortcut to convert HTML TO razor syntax .

What are the specifics of using web api what added advantage i would have ?