people like doing things in different ways. but i see two reasons probably:
it has many more options, and lets you do some things that itunes wants to prevent.
and it's bash scriptable.

my question is why not just install it from macports?

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from the man:

OPTIONS
-a artist Specify ID3 artist name
-c comment Specify ID3 comment
-g genre Specify ID3 genre (use -G for genre list)
-l album Specify ID3 album name
-n track Specify ID3 v1.1 track number
-t title Specify ID3 track title
-y year Specify ID3 copyright year

-G Display valid genres
-h Display this help page
-x Display technical attributes of the MP3 file
-r a|m|v Report bit rate of (VBR) files as:
a - Average bit rate (float)
m - Median bit rate (integer)
v - Simply use the word 'Variable' (string) [default]

-i Edit ID3 tag interactively
-d Delete ID3 tag (if one exists)
-f Force processing if file is not a standard MP3
-F Full Scan (see man page for details)


-p "FORMAT_STRING" Print FORMAT_STRING with substitutions

Conversion Specifiers

%f Filename without the path (string)
%F Filename with the path (string)
%k File size in KB (integer)

%a Artist (string)
%c Comment (string)
%g Musical genre (string)
%G Musical genre (integer)
%l Album name (string)
%n Track (integer)
%t Track Title (string)
%y Year (string)

%C Copyright flag (string)
%e Emphasis (string)
%E CRC Error protection (string)
%L MPEG Layer (string)
%O Original material flag (string)
%o Stereo/mono mode (string)
%p Padding (string)
%v MPEG Version (float)

%u Number of good audio frames (integer)
%b Number of corrupt audio frames (integer)
%Q Sampling frequency in Hz (integer)
%q Sampling frequency in KHz (integer)
%r Bit Rate in KB/s (see also '-r')

%m Playing time: minutes only (integer)
%s Playing time: seconds only (integer)
%S Total playing time in seconds (integer)

%% A single percent sign