Bootstrap.css why write (col-lg-6 col-sm-6) when you can just (col-sm-6) and everything works the same way?

Question

css

I often meet similar examples
example
col-lg-3 col-md-3 col-sm-6 col-xs-6
and why not
col-md-3 col-xs-6

Answer 1

apparently, there were a lot of changes during the development process and in the final they forgot to remove unnecessary classes

Answer 2

depends not only on the size of the screen on which viewing is supposed to be, but also on DPI. Neglecting these parameters may make it impossible to use the resource on some devices (it will be very inconvenient)