Let's say you have 1GB RAM. You're running program that occupies 600 MB. Now this program wants to launch second small program that occupies 1 MB.

You're doing fork + exec.

If you're overcommiting, fork will not reserve another 600 MB, and exec immediately after fork will cause total system usage to be 601 MB.

If you're not overcommiting, that fork will fail, because total memory consumption will be 1200 MB which is more than 1GB. That somewhat restricts program design.