SERVER:~ # fuser -uc /var/log/var/log:             6824(daemon)  9902(root) 14011(root) 14084(root) 14208(lp)SERVER:~ # fuser -uc /var/log 2>/dev/null  6824  9902 14011 14084 14208SERVER:~ # SERVER:~ # The question: is it because of historical reasons or are there any technical reason for it?
Why does fuser use the STDERR?
fuser
The fuser utility is bound by standards as below:The rational for this to have diagnostic information output to stderr. This not only includes errors but anything that is not primary data output of the utility.While this is useful data, if it is included in the stdout, then parsing the output will become increasingly difficult.If stdout is being redirected or filtered, stderr still keeps printing to the terminal.According to the IEEE Std 1003.1, 2004 Edition, the fuser utility:The *fuser* utility shall write to standard error additional information about the  named  files indicating how the file is being used.   STDERRThe fuser utility shall write diagnostic messages to standard error.     The fuser utility also shall write the following to standard error:The pathname of each named file is written followed immediately by a colon.For each process ID written to standard output, the character 'c'  shall be written to standard error if the process is using the  file as its current directory and the character 'r' shall be  written to standard error if the process is using the file as  its root directory. Implementations may write other alphabetic  characters to indicate other uses of files.When the -u option is specified, characters indicating the  use of the file shall be followed immediately by the user name,  in parentheses, corresponding to the process' real user ID.  If the user name cannot be resolved from the process' real user  ID, the process' real user ID shall be written instead of the user name.When standard output and standard error are directed to the same file,  the output shall be interleaved so that the filename appears at the  start of each line, followed by the process ID and characters  indicating the use of the file. Then, if the -u option is  specified, the user name or user ID for each process using that  file shall be written.A <newline> shall be written to standard error after the last  output described above for each file operand.ReferenceIEEE Std 1003.1, 2004 Edition