A characteristic of Fretless Bass is its smooth, sweet and creamy “bwoww” sound. To me it often sounds like a lonely sad sigh. When the bass isn't fretted, it allows for a smooth sliding between the notes instead of jumping between them because frets are like little road bumps. Fletless bass has no road bumps and this increases its potential. But this beautiful and delicious silky sound has its price - fretless bass is much harder to play because the player has to find the right notes without the "orientation" provided by the frets on the standard fretted bass. Also, getting a good tone / intonation on fretless bass takes more effort. I am talking about bass players of master class here. Most of the metal bands that have used fretless bass so far are death metal bands, especially technical and progressive death metal. This may surprise you because fretless bass has a smoother, softer and warmer sound. But here's the bonus: when the guitars play really heavy power chords, the fretless bass is sliding between the notes emphasized by the guitars creating a tonal contrast. This contrast has a very powerful effect and this type of bass playing is very creative. It's probably the most important aspect of fretless playing in metal. But songwriting gets even more demanding.