Why custom controllers was provided by salesforce if we can achieve same funtionality with standard controller and extensions?

[SELECT MailingStreet, MailingCity, AccountId FROM Contact WHERE AccountId IN : acctIds] In this query we will retrieve Contacts based on Account IDs,  I want to implement the same functinality using Custom Object  Where Parent object is Campsite__c and Child Object is Campsite_Reservation__c. Can any body write the query? And anyone explain when to use __r and  __c