Transparent thread are made from very thin plastic. I use it sometimes when i want to sew something but I’m not sure what color to use.
The example case is when i want to sew white pearl on black ribbon. If i used black thread, it will be visible on white pearl though a bit. If I used white thread, it will be visible on black ribbon though a bit. Using transparent thread is a good option in this case.
Another example, i saw some ribbons have beads sewn on to. The thread used is transparent. It won’t ‘ruin’ the color of the beads as well as the ribbon itself.