In my opinion, one reason could be some highly technical users may want full control over how they want their browsers to behave. Javascript is one part of browser behavior which some users may wish to control. Thus - it is a question of choice. Sometimes - some Javascript could be too annoying for some choosy techie users and they may wish to turn it off.
Thus - in addition to security and privacy, it is a choice that browsers offer to end users. However it is true that nowadays most sites hardly function without JS. But still there could be those plain old vanilla HTML sites around that may do without any client side scripting.
Regards