A user agent (UA) on the WEB is generally for declaring browser information and names of crawlers when connecting to a server. Although some access controls are specified by specifying a user agent such as robots.txt, a user agent can easily impersonate, so a system that trusts this 100% should not be created.
The main use is to identify the crawlers of google, aggregate the dates and times of visits to the crawlers ,
and identify accesses from iPhones and iPads to count the number of accesses individually or switch display data. And so on.
● Reference information
List of user agents used by Google
List of general user agents