You have many benefits, if you use the devise detection. You can increase the activity of web site and complete your existing web site analysis tools. You can improve your web site, if you optimize processes and design of your page for different devices. You improve also the productivity of the developer, if your reduce the time for testing. It will be much more easier for them to see the traffic on your websites.
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Business
- premium content for users of certain devices
- link to documentation for appropriate platform
- advertising - device specific promotions
User Experience
- send old browsers to a basic version and mobile users to the mobile web site
- mobile content vs tablet content vs desktop content vs wearable content vs TV content
- images can be optimized server-side to save bandwidth and provide a better UX
- delivering lighter experience to less capable devices
- determine the optimal video format to send
Technical
- protection against js executing crawling bots
- custom content serving for special vendors
- to workaround a specific browser bug
- detecting click-to-call scheme support
- Stop asking your clients: "What browser are you on?"
Others
- collecting UAs for analytics/statistics reporting
- spambot detection
