Add benefits of abstraction to documentation

docs/contracts.rst

@@ -1056,7 +1056,8 @@ As an exception, a state variable getter can override a public function.
 Abstractions
 ************
 
-An abstraction type describes a contract but does not provide a full implementation of the contract. An abstraction is implemented as either an abstract contract or interface.
+An abstraction type describes a contract but does not provide a full implementation of the contract. An abstraction is implemented as abstract contract or interface.
+An abstraction allows you to decouple contract definition from implementation providing better extensibility and self-documentation.
 
 .. index:: ! contract;abstract, ! abstract contract
 
@@ -1085,7 +1086,8 @@ Such contracts cannot be compiled (even if they contain implemented functions al
         function utterance() public returns (bytes32) { return "miaow"; }
     }
 
-If a contract inherits from an abstract contract and does not implement all non-implemented functions by overriding, it will itself be abstract. 
+Abstract contracts can implement most of a contract, but leave some methods abstract, facilitating patterns like template method and removing code duplication.
+If a contract inherits from an abstract contract and does not implement all non-implemented functions by overriding, it will itself be abstract.
 
 .. index:: ! contract;interface, ! interface contract