The threshold for potential instant noise-induced hearing loss is 120 dB SPL at the ear. A rifle can exceed 170 dB SPL at a distance of one meter[1]. When you're shooting a rifle, it's going to be cradled firmly against the shoulder -- a lot closer than 1 meter. Even the lowly .22lr fired from a handgun can exceed 160 dB SPL[2]. A suppressor can knock that down about 30 dB. 130 dB is still a lot, but it's out of the 'will-immedately-and-permanently-damage-your-hearing' range at least, and makes further hearing protection (earmuffs, earplugs) that much more effective.
tl;dr - A suppressor is safety equipment.
1. https://en.wikipedia.org/wiki/Sound_pressure#Examples_of_sou... 2. https://en.wikipedia.org/wiki/Suppressor#Effectiveness