Update Span.md

docs/Span.md

@@ -72,5 +72,11 @@ public struct Span<T> {
 In addition, we will provide a read-only version of Span\<T\>. The ReadOnlySpan\<T\> is required to represent slices of immutable and read-only structures, e.g. System.String slices. This is discussed below.
 
 ## Scenarios
+Span\<T\> is a small, but critical, building block for a much larger effort to provide .NET APIs to enable development of high scalability server applications.
+
+The .NET Framework design philosophy has focused almost solely on productivity for developers writing application software. In addition, many of the Framework’s design decisions were made assuming Windows client-server applications circa 1999. This design philosophy is big part of .NET’s success as .NET is universally viewed as a very high productivity platform.
+
+But the landscape has shifted since our platform was conceived almost 20 years ago. We now target non-Windows operating systems, our developers write cloud hosted services demanding different tradeoffs than client-server applications, the state of art patterns have moved away from once popular technologies like XML, UTF16, SOAP (to name a few), and the hardware running today’s software is very different than what was available 20 years ago. 
+
+When we analyze the gaps we have today and the requirements of today’s high sclae servers, we realize that we need to provide modern no-copy, low-allocation, and UTF8 data transformation APIs that are efficient, reliable, and easy to use. Prototypes of such APIs are available in [Corfxlab repo]( https://github.com/dotnet/corefxlab), and Span\<T\> is one of the main fundamental building blocks for these APIs.
 
-TODO