When you’re playing a fretless instrument, such as a fretless bass guitar or a violin, it’s easy to be slightly sharp or flat as the note depends on the exact placement of your fingers. While musicians are careful to play with as much precision as they can, it can be fun to play off the note a bit, whether it be in a vibrato (the rapid pulsing or wavering of a tone), in a glissando (a slide upward and downward between notes), or through an intentional (or unintentional) departure from playing exactly on the note. With a fretted instrument, the fret bars keep your playing to those specific notes for that fret. They are certainly easier to play and provide more precision, but they limit the player to only those specific notes.
I recently bought a fretless bass and am having a great time playing with these dynamics. The ability to play expressively when I’m not limited by frets has given me a newfound sense of freedom and has allowed me to play in a way I’ve never played before. It has also made me think about the continuous range of tones being symbolic of how people live their lives.