[]byte is crucial if you need to frequently modify the contents. The classic example is building a string with a loop that appends a new character on each iteration. Using += to concatenate strings inside such a loop will be slow; the Right Thing To Do is to preallocate a []byte with sufficient capacity, append to it in a loop, and then (if necessary) convert to a string only at the end.
Isn't there a premade library that will abstract these optimizations away? e.g. a rope library. Looks like there's a few libraries out there that implement this data structure. (https://en.wikipedia.org/wiki/Rope_(data_structure))
Yes, it's just []rune, which has just a bit of special sauce in the runtime that allows you to convert it directly to a string [1], and some functions that convert UTF-8 to and from it. However rune is a 32-bit number sufficient for storing a Unicode code point. Generally you're dealing in UTF-8 and prefer to just pass that through code. I think I've only ever use a []rune once, for when I was doing some very heavy-duty Unicode-aware code, and I still ended up refactoring it into using UTF-8 directly in the end.
[1]: https://play.golang.org/p/5DEzw85J5Ob Note this is a conversion; the string is UTF-8 and the runes are 32-bit ints, so this creates a new string.
