yocalhost is an http development server that simulates latency and bandwidth limitations.
Evaluating a network client against a remote host is subject to the whim of the rest of the internet. Especially if your client involves many requests, it can be hard to get consistent measurements.
Evaluating against a naive localhost server, where latency and bandwidth are unreasonably fast, can draw a false picture.
With yocalhost, you run a local http server, but specify artificial bandwidth and latency limitations. This allows you to measure realistic(ish) implications of latency and bandwidth on your code in a reproducible way.