As a musician, you possibly have custom ones, but for anyone else who’s not an audiophile, you can get decent ones that mostly preserve the sound while lowering the volume for around $15-20. A godsend for concerts.
Many of the rock players are moving to in-ear monitors, which block everything but replace it with your monitor mix, received via wireless.
On the other hand, some band mates, and my kids, play classical. Their needs are a lot more critical in terms of perceiving their tone quality. A benefit of the fitted plugs is that you can get the capsules with less attenuation, which means you're still safe in something like a classical ensemble, but without losing too much.
Maybe necessary for movie theatres too. Contemporary volume levels are abusive (imo).