BOL 2000 says:

Using ROWS_PER_BATCH

If the -b switch or BATCHSIZE clause is not used, the entire file is sent to SQL Server and the bulk copy operation is treated as a single transaction. In this case, the ROWS_PER_BATCH hint or ROWS_PER_BATCH clause can be used to give an estimate of the number of rows. SQL Server optimizes the load automatically, according to the batch size value, which may result in better performance.

BOL 2005 says

ROWS_PER_BATCH = rows_per_batch

Indicates the approximate number of rows of data in the data file.

By default, all the data in the data file is sent to the server as a single transaction, and the number of rows in the batch is unknown to the query optimizer. If you specify ROWS_PER_BATCH (with a value > 0) the server uses this value to optimize the bulk-import operation. The value specified for ROWS_PER_BATCH should approximately the same as the actual number of rows.