h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Another important advantage of the fretless bass is the opportunity for the player to step outside the confines of the well-tempered tuning.

(Skip this paragraph if you are already familiar with the well-tempered tuning) Well tempered tuning is the very clever compromise that allows instruments with fixed tunings, such as fretted and keyboard instruments, to play, in tune, in any possible scale. The means of achieving this is to squash in each pitch to the best compromise for all possible keys. The upshot of this is that playing a fretted bass you are sort of in tune in all keys, but exactly in tune in none.

The fretless bass gives the player the chance to play with correct intonation. This can produce beautiful harmonies, but is naturally most effective in pieces of a slower tempo.

Try it out by playing harmonies below a steady fixed tone and wiggle your finger about (slowly) and see if you can find the intonation that makes that fifth or major third really sing out.