Find the byte sum of a file to verify its integrity in DOS.

Such a requirement in case of communication as well as storage (storage can be considered as communication where source is current you and destination is a later you). Communication is almost always lossy, and can introduce errors into data. A measure like this helps to verify the correctness of data (but cannot guarentee that it is correct). More details here.

Enter Filename whose byte sum is required.

Checksum is the sum of bytes in the file.