Blind copying of C, just like ratchet freak said in his comment

The vast majority of “language designers” these days have never seen anything but C and its copies (C++, Java, Javascript, PHP, and probably a few dozen others I never heard of). They have never touched FORTRAN, COBOL, LISP, PASCAL, Oberon, FORTH, APL, BLISS, SNOBOL, to name a few.

Once upon a time, exposure to multiple programming languages was MANDATORY in the computer science curriculum, and that didn’t include counting C, C++, and Java as three separate languages.

Octal was used in the earlier days because it made reading binary instruction values easier. The PDP-11, for example, BASICALLY had a 4-bit opcode, 2 3-bit register numbers, and 2 3-bit access mechanism fields. Expressing the word in octal made everything obvious.

Because of C’s early association with the PDP-11, octal notation was included, since it was very common on PDP-11s at the time.

Other machines had instruction sets that didn’t map well to hex. The CDC 6600 had a 60-bit word, with each word containing typically 2 to 4 instructions. Each instruction was 15 or 30 bits.

As for reading and writing values, this is a solved problem, with a well-known industry best practice, at least in the defense industry. You DOCUMENT your file formats. There is no ambiguity when the format is documented, because the document TELLS you whether you are looking at a decimal number, a hex number, or an octal number.

Also note: If your I/O system defaults to leading 0 meaning octal, you have to use some other convention on your output to denote hexadecimal values. This is not necessarily a win.

In my personal opinion, Ada did it best: 2#10010010#, 8#222#, 16#92#, and 146 all represent the same value. (That will probably get me at least three downvotes right there, just for mentioning Ada.)

They get it from C. Why copy? Because the base implementation of all 3 is in C. Python’s default implementation is CPython. Ruby was originally built in C as well. Javascript is the most interesting case here. It’s run in the browser. Care to guess what the first web browser was written in?

So why would all three of these languages be implemented in C? Because they all originate on UNIX systems. So it’s a case of convention being driven by ecosystem. Perl does this as well. Lua likely would if Lua used integers rather than doubles.

So it’s a question of the environment of these languages being written in C so they take their conventions from C. A good supporting corollary is Visual Basic which uses &O instead. As far as needing it, it seems to be more of a leaky abstraction turned convention than anything else.

There is a value to consistency. If you can’t reliably determine how a number is going to be translated you will have real problems using a value in different contexts.

It also means you don’t have to write your own parser. There is great value in using well tested library routines.

Also if you don’t support the leading 0 syntax you don’t have a simple way to write octal values.

While we don’t depend as much on octal numbers as we once did, they are still of value. While the same results can be obtained with hexadecimal numbers, in some contexts octal is easier to understand.

So far I have only seen one use for leading zeros in decimal numbers. That is in the display and entry of fixed length decimal fields like identification numbers. It has been years since I have seen fields like that with a leading zero. While this reduces the available values by 10%, it eliminates the problem that users often leave off the leading zeros when entering them.