FIx intro to manual chapter on types

doc/src/manual/types.md

@@ -16,10 +16,11 @@ of function arguments to be deeply integrated with the language. Method dispatch
 detail in [Methods](@ref), but is rooted in the type system presented here.
 
 The default behavior in Julia when types are omitted is to allow values to be of any type. Thus,
-one can write many useful Julia programs without ever explicitly using types. When additional
+one can write many useful Julia functions without ever explicitly using types. When additional
 expressiveness is needed, however, it is easy to gradually introduce explicit type annotations
-into previously "untyped" code. Doing so will typically increase both the performance and robustness
-of these systems, and perhaps somewhat counterintuitively, often significantly simplify them.
+into previously "untyped" code. Adding annotations serves three primary purposes: to take advantage
+of Julia's powerful multiple-dispatch mechanism,  to improve human readability, and to catch
+programmer errors.
 
 Describing Julia in the lingo of [type systems](https://en.wikipedia.org/wiki/Type_system), it
 is: dynamic, nominative and parametric. Generic types can be parameterized, and the hierarchical