Because it use the isolated sandbox for development activities.
You can develop the office and sharepoint application in browser itself and doesn't require to install any development tools.
How to activate the "Napa" tool for your Office 365 site.Here I am using SharePoint 2013,if you are using the Office 365,SharePoint 2010 the visual steps may be slightly differ from this.
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Napa Office 365 Development tool |
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Napa Office 365 Development tool |
Step 3
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Napa Office 365 Development tool |
Step 4
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Napa Office 365 Development tool |