when concatenating strings, stringbuffer can be orders of magnitude faster. one c# example from google search "stringbuilder vs string benchmark" from codinggame.com measures 2.5 minutes for string += and 99ms for stringbuilder.

Maybe for Python a stringbuilder is faster, but for JavaScript, += is faster.

https://jsperf.com/javascript-concat-vs-join/2

Don't assume a certain way of coding is faster because you read it on the Internet, actually profile your code.

sry i meant stringbuilder, not stringbuffer.

They allow you to change the complexity from O(n^2) to O(n).