Some instruments (for example the guitar, the bass guitar, and the xylophone) have written music in a typical clef, however the note written and the note played are an octave lower or higher depending on the instrument. There are octave clefs that can be used in place of having an instrument transpose by octave, but are typically not used. Is there a reason why transposing by octave is more preferable then using an octave clef?
In practice there is little difference between using an octave clef and a normal clef for these "octave-transposing" instruments. An instrumentalist playing these instruments need not even think about the fact that the music sounds in a different octave to that written; although, of course, players and composers/arrangers should know that the sounding pitch is different to the written pitch. The octave clef simply makes explicit what is implicit to instrumentalists and composers/arrangers.
If, on the other hand, you are asking about why these instruments use octave transposition at all, it is usually for one of three reasons:
Either way, this is simply a practical shift of pitch used to allow music to be notated most easily on a particular stave, without using lots of ledger lines; it isn't as significant in practice as non-octave transpositions.
Just as a final note: I have seen intriguing examples of guitar music written at sounding pitch, on dual treble and bass clefs, in the same way as piano music. One well known example is the orchestral Electric Guitar part for Stockhausen's Gruppen for Three Orchestras.
It's down to the range of the instrument compared with the range of the clefs. With guitar, the lowest note is traditionally an E. This E would come on the 1st ledger line below the bass clef. Putting the open top string on the treble clef stave,bottom line. That's before we go to the rest of the notes up that string. Playing on two staves isn't easy (ask a pianist !), so it makes sense to use just the treble clef as then the lowest note is only 3 ledger lines down,(just under) and this gives headroom for more of the higher notes. Yes, it's an octave out, shown usually by the little '8' by the clef sign, but guitarists don't think about it much, if at all. This makes the guitar a transposing instrument, which happily uses an ordinary,better-known clef, without having to resort to an alto or tenor clef, which could put everything in a good place, but folks are more familiar with the standard treble clef stave.
The octave clef, with a low '8' should be used, and on quality music, is. However, guitarists just become programmed, and 'know' where to go.
Actually, I've just looked through some of my library - of classical through to contemporary - and there are only a few with that octave clef.Yes, we must just have got used to it !!
Not at all sure whether this answers your question...
Actually I think that at least in one respect your question starts at the wrong end: this is no property of the instrument, but a convention of how it is notated. Double bass and contrabassoon sound an octave lower than they are notated. The notation is already in the bass clef, so while one could use ( impracticable many ledger lines or) a transposing clef, it is clear for any player as it is. I have never seen a down- transposing tenor clef which would be appropriate for the high register. Same applies in the other direction for piccolo flute.
This leaves the transposing key available for further extremes (technically impossible for lower direction of contrabasson, unsure about higher direction of piccolo).