there are a few reasons. like tiniuclx said, it just sounds different. a downtuned guitar-scale string is going to have a very different tone than a low bass-sc

there are a few reasons. like tiniuclx said, it just sounds different. a downtuned guitar-scale string is going to have a very different tone than a low bass-scale string. like you say, low notes can sound very muddy when distorted. interestingly enough, people usually EQ out most of the low-end frequencies when they play in lower tunings. seems counterintuitive, but it makes for a very interesting sound to my ear. anyways, the main reason to have extra strings is to have a greater range. if you just want to play very low notes, you can downtune a regular six-string guitar pretty far, you could just get a bass, or you could get a baritone guitar for something in between. here's an example of a riff that uses almost the entire range of an eight-string guitar: https://youtu.be/v9RIJ4XlUPU?t=66 https://youtu.be/v9RIJ4XlUPU?t=66