If there is a CRC mismatch, then it would be nice to know which chunk was corrupted. If, for example, the CRC mismatch is in a text chunk, then it would not be a great loss to the user and the image would most likely still be viewable. If, on the other hand, the CRC mismatch is in the image data chunk, then the image on the screen will be garbled to some degree. The user might in that case want to re-request the image from where they got it from.
Using the chunk type also fixes an unused field warning.
If there is a CRC mismatch, then it would be nice to know which chunk was corrupted. If, for example, the CRC mismatch is in a text chunk, then it would not be a great loss to the user and the image would most likely still be viewable. If, on the other hand, the CRC mismatch is in the image data chunk, then the image on the screen will be garbled to some degree. The user might in that case want to re-request the image from where they got it from.
Using the chunk type also fixes an unused field warning.