If you develop multiple websites on your local machine, you distinguish them by using different domain names for each of them. You might have site1.dev and site2.dev.

There are a variety of methods to handle this, but commonly you add a record to your system's host file to resolve these domain names to the ip address 127.0.0.1 (your local computer).

Using site1.til.io and site2.til.io instead eliminates having to manage that host file (which is a minor and repetitive annoyance).