The guitar is a transposed instrument. It sounds an octave lower than notated. Sometimes it is shown by using a “Treble down 8” clef and sometimes not. For solo guitars and ensembles with guitars only, it isn’t really necessary – the guitarist know that the instrument is transposed, and the information (on the cover) that the music is for guitar simply implies, that the score will sound an octave lower than notated. But in mixed ensembles – especially if a conductor is involved, it make sense to use the “down 8” clef. It reminds the conductor that the pitch of the guitar is an octave lower than notated.
Old scores for guitar never uses treble down like this old Scott example:
Newer publications come in both versions:
For me as a guitarist, it doesn’t make any practical difference if the clef is notated one way or the other. As an engraver, I avoid using it unless it is for mixed ensembles – it is redundant information and aesthetic it is hard to find arguments that the little added “8” improve the look of the clef in any way.