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.
premium content for users of certain devices
link to documentation for appropriate platform
advertising - device specific promotions
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
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?"