The Razor syntax was originally developed for ASP.NET Web Pages, which is the simplified programming model that WebMatrix supports. Then someone thought it would make quite a groovy View engine for ASP.NET MVC, so it has been adopted there too. The syntax is identical, regardless whether you are developing MVC or Web Pages.

I wouldn't recommend developing MVC apps with WebMatrix. Web Pages (and WebMatrix) support a different compilation model to MVC. It's the one that works for Web Forms Web Sites as opposed to Web Applications. OK, now to the specific questions:

yousaid

(a) If I already have VS 2010, why should I want WebMatrix?

You might want to have a play. You wouldn't use WebMatrix as a serious part of your day job unless you wanted to use it for prototyping (downloading and using a CMS as a proof of concept, for example).

yousaid

(b) Other than when I want to use Helpers, when should I use WebMatrix if I already have VS201?

You don't even need to use if for the helpers. They are available via NuGet.

yousaid

(c) Assuming I develop APP1 using WebMatrix and then  develop same APP1 using VS2010 MVC (same Razor engine) will there be any differences in the Syntax

Nope.

yousaid

WebMatrix is for PHP developers moving to .Net and MVC is for seasoned .Net developers.

I'd say that WebMatrix is for anyone wanting to get started with ASP.NET, whether they know anything about web devleopment or not.

MVC is for more than just seasoned ASP.NET developers. RubyOn Rails, Java Struts, CakePHP, etc, etc - there are loads of MVC frameworks, and experienced users of any of them can migrate to ASP.NET MVC quite easily.