Why an abrupt start or end sounds wrong
Audio that begins or ends at full volume instantly, with no ramp, creates the same kind of audible discontinuity discussed with cuts and clicks — a sudden jump from silence to full amplitude (or vice versa) can produce a mild pop or, at minimum, an abruptness that reads as unpolished even when it's not technically a click. Fades solve this by ramping the volume gradually from zero at the very start, and back to zero at the very end, so there's no instant jump anywhere.
Fade curve shape actually matters, not just duration
A linear fade (volume changing at a constant rate) is the simplest option, but it doesn't always match how loudness is actually perceived — human hearing perceives loudness roughly logarithmically, so an exponential or S-curve fade can sound smoother and more natural than a straight linear ramp, particularly for longer fades. For short, quick fades used just to avoid a click at a cut point, the curve shape barely matters, but for a deliberate, several-second fade-out at the end of a track, the curve shape is a genuine creative choice worth trying a few options for.
How long a fade should be depends entirely on context
A fade used purely to eliminate a click at an edit point only needs to be a handful of milliseconds — long enough to avoid the discontinuity, short enough to be imperceptible as an intentional fade. A musical fade-out at the end of a song is a completely different, deliberate creative element that typically runs several seconds, sometimes much longer, and is meant to be heard and felt as part of the piece rather than hidden.
Fades are cheap insurance for any edited audio
Adding a very short fade-in and fade-out to essentially any trimmed or cut audio clip — even ones that don't seem to need it — costs nothing perceptually when done briefly, and it eliminates a whole category of potential click or pop issues at the boundaries. This is why professional audio editing tools often apply a tiny default fade automatically on any cut or trim, treating it as a safety measure rather than an optional polish step.