You can manually increase or decrease the size of a datafile using the ALTER DATABASE statement. This enables you to add more space to your database without adding more datafiles. This is beneficial if you are concerned about reaching the maximum number of datafiles allowed in your database.
For a bigfile tablespace you can use the ALTER TABLESPACE statement to resize a datafile. You are not allowed to add a datafile to a bigfile tablespace.
Manually reducing the sizes of datafiles enables you to reclaim unused space in the database. This is useful for correcting errors in estimates of space requirements.