A disk file, for example MASTER FILE, can be opened as I-O, 
which means records from the disk will be accessed, read, 
changed, and rewritten.

Without knowing what field to rewrite we cannot use rewrite
the field in a record.Thats why we read the file first and
then rewrite the same

Hi,

For Rewrite, the file has to be opened in I-O mode. REWRITE 
is used to update records. Rewrite statement will update 
the last read record. Hence, we have to read the record to 
be updated first. Hope this clarifies.

Rewrite is nothing but Update so if you want to 
modify/changes in the record you should read the record 
first, which you want to 
then you  can rewrite it. I hope it will clarify your 
doubts.