Devs don't know Ops
If left to their own, Devs will produce a system that works for now. But they may well not think to add any of the things that the Ops half of the equation are there for:
* Backups that are reliable and tested and will work when they are needed. (321 rule)
* Monitoring that will warn you _before_ things fail (Out of disk space? Again?)
* Alerting for when things do fail.
* System updates and patching - 'cause new vulnerabilities come along all the time (e.g. heartbleed, etc.)
* Security: no, you can't just use one log-in for everything.
* Capacity planning
* High availability
* Scaling
etc
The main benefit of DevOps is to get the Devs thinking about how their code will run in production _before_ the final deployment.