Decay is a fundamental characteristic of any acoustic space. A long, smooth decay sounds spacious and lush (concert hall). A short, controlled decay sounds tight and precise (recording studio). An uneven decay — where some frequencies ring much longer than others — sounds unnatural and problematic.
In mixing, decay time is primarily controlled by reverb plugins. In room acoustics, it is controlled by absorption (reduces decay) and diffusion (scatters reflections for even decay). Measuring RT60 (time for sound to decay 60 dB) is the standard decay metric.