Because  H* gives you a single string of an unlimited number, necessarily even, of hex characters, but  (H2)* gives you an unlimited number of groups of pairs (or in the case of  (H4)* quartets). The parentheses do grouping in pack/unpack templates. The groups can be quantified. BTW: This is susceptible to experimentation:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $octetstr = qq{\xab\xcd\x00\x1d\x94\x56};
 ;;
 my @ra = unpack 'H*', $octetstr;
 dd \@ra;
 ;;
 @ra = unpack '(H2)*', $octetstr;
 dd \@ra;
 ;;
 @ra = unpack '(H4)*', $octetstr;
 dd \@ra;
"
["abcd001d9456"]

["ab", "cd", "00", "1d", 94, 56]

["abcd", "001d", 9456]
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Because an even number of hex digits (H) is needed.

Alexander

Because bytes (pompously aka. octets) consist of 2 hex digits dec:0 -> 0x00 dec:255 -> 0xff.

The octet term is commonly used in networking, RFC's, etc. It designates an 8-bit unit.

A byte is generally 8 bits, too, although some historical computers had 9-bit bytes (and 36-bit words). There might be some uses for wide character definition today as well, as there are architectures where memory is word-addressable.

I suppose the writers of standards and documentation prefer not to get sidetracked with moot issues in terminology, hence the unambiguous "octet".

The byte (/ˈbaɪt/) is a unit of digital information that most commonly consists of eight bits. Historically, .... The modern de-facto standard of eight bits, as documented in ISO/IEC 2382-1:1993, is a convenient power of two permitting the values 0 through 255 for one byte.5 The international standard IEC 80000-13 codified this common meaning. Many types of applications use information representable in eight or fewer bits and processor designers optimize for this common usage. The popularity of major commercial computing architectures has aided in the ubiquitous acceptance of the eight-bit size

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