When recording instruments and vocals, some sounds can be too loud or faint. Some equipment handles overly loud passages poorly. To manage that, effects such as compressors and limiters automatically adjust the loudness of sounds. Limiters prevent loud sounds from overloading the equipment. Compressors can make music more loud overall.
Musicians often need to sculpt the frequency response of their music. You can boost bass, trim highs, and otherwise tailor the sound. Some effects units employ active filtering, providing wah-wah type modulation, phase shifting and related effects.
Most musicians use echo and reverb to make the music sound fuller or enhance the mood. These effects mix the original sound with a delayed copy. Since delay time can be varied from milliseconds to seconds, you can obtain a range of effects. You can create echo, reverb and dramatic effects such as flanging and chorus. The early 1980s saw a lot of music produced using then-new digital delays. Modern rack-mount units offer these classic effects and more sophisticated versions.
Vacuum tubes have made a comeback. Though transistors are more efficient, tubes amplify sound differently, especially at the extremes of loudness. Musicians have sought the classic tube sound. Equipment makers have responded with a range of rackmount tube amplifiers, preamps and sound effects. They also offer emulators, digital circuits that mimic the warm tube sound.
Rackmount effects are rarely controversial, but Autotune created a stir when it was introduced in the late '90s. It's a computerized system that senses pitch errors in vocals and automatically fixes them. It became very popular, though music critics and musicians have derided it as a crutch for bad singers. Many working musicians use it as a safety net for their vocals in live performance. Controversy aside, Autotune has become a fixture in all genres of pop music.