Understanding semantic web need - info request

Hi...
   I am new to OWL and am trying to understand the use cases where an
ontology language is more efficient than the turing complete languages like
C, Java.

   I also tried to find advantages of semantic web like OWL through
Stackoverflow and other sites but only got very general information and
could not arrive at specifics.
Can someone explain me  the need for OWL likes' with a few helpful use
cases.

http://www.w3.org/TR/owl2-primer/#Datatypes..
   Having few questions going through the link ; if we need to provide all
such negative assertions as well during the development( like to get proper
class of "Felix" while querying the reasoner), would it not increase dev.
effort and also less efficient than closed world system.

Requesting response,
Thanks.