Why both "Value" and "Field" in column customization

Am I missing something or is the Field box redundant?

Within the list view you can define colums, which do not write to a tag-field, just for displaying purpose.
For example:
Name: Covers; Value: %_covers%
Name: Codec; Value: %_codec%[' ('%_tool%')']
Name: VBR; Value: [$left(%_vbr%,1)]
Name: Bitrate; Value: [%_bitrate%' kBit/s ']
Name: Samplerate; Value: [%_samplerate%' Hz']
Name: Duration; Value: %_length%; SortBy: %_length_seconds%; Numeric: yes
Name: Filesize; Value: %_file_size_mb%' MB'; SortBy: %_file_size_bytes%; Numeric: yes

DD.20140622.0610.CEST

Oh, so field actually writes to the tag, while value is just for display.
Makes sense now that you've said it. I usually edit tags in the panel, not columns, so I didn't think of it.

Editing is locked if Field is not defined.