I wanted to know that why Timer job is deployed at web application level only
closed as unclear what you're asking by Mohamed El-Qassas MVP♦ Oct 10 '17 at 9:55
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This is because the SharePoint timer job utilizes SP Timer services under the services on server.
To access this it requires to be deployed at farm level as site and web level features cannot access the SPTimer service object.
Hope this helps
Thanks, Happy Coding.