why use webservices or wcf services how can they useful in our web application?

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WCF

in real time scenario when the webservices or Wcf services are useful.what is the need of usage of our functions or methods usage in other platform application. iam thinking that if i have a web application and if want it to convert to the mobile app android or what ever in that case by writing the methods(webservices or wcf services) for my web app and we can use the same functionality in the mobile app without writing the business logic and datalayer code . this is my assumption i don't know this is write or wrong because i am new to webservices can any explain anyone about web or wcf services(realtime usage especially).


Regards
Hemath