Every time you use your browser to access a website a User-Agent header is sent to the respective server.
Detecting user agents on the server can be useful for many reasons.
1. Browsers Quirks – Despite the standardization in browsers, there will remain some quirks in various browsers that you will need to iron out on a regular basis.
2. Personalize Content – It may be required to deliver different type of content depending on the browser type (although it is usually not recommended); whether mobile or otherwise.
3. Illegal Access – Prevent bandwidth hogging bots and poorly programmed clients from downloading your content.