I have a lots of datafiles on SYSTEM (user) tablespace, due to activate audit logs for a while but didn't need that anymore, I have to free space of the hard disk of the server where the Database is hosted, can I drop some of them or use the REUSE option, in that case what are the advantages of using REUSE?
The reuse option simply allows an existing file to be used in a tablespace. It will, as far as I remember, be formatted for use.
You can drop data files but it depends on your version of Oracle. Check the Alter tablespace drop ..." sql command in the SQL Language Reference manual for your version.
This might help https://docs.oracle.com/cd/B28359_01/server.111/b28310/dfiles006.htm.
By the was, SYSTEM tablespace is definitely not for users. That way, horrors lie!