Why build apps for SharePoint 2013?

Apps:
 
Apps for SharePoint are easy-to-use, lightweight web applications that integrate popular web standards and technologies to extend the capabilities of a SharePoint website.
 
Why build apps for SharePoint?

There are many great reasons to build an app for SharePoint:
  1. A familiar programming model, plus popular technologies and languages
  2. Has multiple options for hosting
  3. A familiar web experience, yet more flexible
  4. Integration with apps for Office
  5. Reach users through the Office Store and the App Catalog

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Referenece: MSDN