Jackie - the commit command is (to me and most everyone who uses Junos) one of the "great" features of all time.
1. You have complete control as to when the changes you make will be actived. Instead of line at a time you queue your changes up and activate them when you are ready.
2. You can pre-stage configurations and then schedule (commit @ date / time.
3. It allows you to revert to a prior version of a configuration with a single command (up to fifty previous).
4. It provides an automatic "undo" capability through "commit confirmed" that is essential when configuring remote sites and dealing with things like ACL's or firewall rules.