Simply put /var creates local variables. These automatically get unset when the script ends. /set creates global variables which stay until you unset them either manually or in the code.
Which one you use depends on if you want/need the information to be saved between each run of the script. If you are using dynamic variables in the format of $+(%,variable,name) have to be set using the /set command, but can be set using /var if you use the format of % $+ variable $+ name
It depends on if the first character in the statement setting the dynamic variable is % or not.