I see some files in there like an
opctrn11.h, but what is the purpose of this directory, if we use
%ORACLE_HOME%/database to store the p/spfile instead?
$ORACLE_HOME/dbs is used to store the spfiles, init files and password files on Linux and Unix systems. But as you pointed out the dbs directory not used on windows servers. So on a Windows servers it is probably not needed.