The most common form of user agent spoofing – that is, changing the HTTP header data that identifies the connecting user to appear as a different user – is certainly for testing and developing webpages. Backend and frontend web devs will spoof their UA to make sure the website behaves the way they want it to from different angles, such as unique devices and locations. Malicious users may make use of tor browsers to mask and change their apparent location for the purpose of, say, using stolen credit card credentials and masking their location to that of the victim’s to avoid detection.