Q: Why do all professional songs fade out at the end?
A: Because hard digital stops sound incredibly jarring and aggressive to the human ear.
If a heavy rock song just instantly violently cuts to absolute dead silence, listeners subconsciously panic thinking their headphones just broke. A smooth, 5-second volume fade perfectly psychologically signals to the brain that the song is intentionally and peacefully ending.
Q: Can I fade in a really loud explosion sound effect so it isn't so aggressive?
A: Yes! Upload the ridiculously loud MP3. Apply a 2-second fade-in. Instead of hitting you like a physical digital brick wall, the sound will rapidly swell up from zero volume. It turns a harsh "bang" into a cinematic "whoosh." Highly recommended.