why the ptc sites are paying us???

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i got this doubt very lately after joining and earning little amount. actually, i want to know now. why the ptc sites r paying us for just clicking on ads? what r those ads? who r the advertisers who provide them these ads? i what way the ptc site will get benefited by being a mediater in between advertiser and clicker? how the advertisers will get benefited by giving us cents for clicking? i'm really confused and need clarity abt this. can any one answer all my questions and clear my mind?