Consider these scenarios:
1. You need to test some new configurations in your my.cnf and you need to reboot MySQL.
2. You want to run some benchmarking on your database.
3. You want to try some new replication ideas that involves creating new replication structure, stopping the slaves, doing failovers, or any ideas related to replication.
And, most importantly, you want to do all of the above without disturbing your production databases.
MySQL Sandbox. A lot of people have written about this gem but until you have use it, you will not realize how simple and yet valuable this tool is.