At a basic level, the most significant benefit for Project Managers or Product Owners (depending on the type of project) is the high quality of the code, or simply, a top-notch app. Thanks to the work of DevOps, people responsible for creating the application have the best conditions for coding and testing, so it can be assumed that they are doing the best possible job in given circumstances.
But we can also go down to more process-related advantages for managers. Let's go back to the pre-DevOps situation and imagine that the app spits a critical error, and users cannot log in.
The programmer quickly fixes what's broken, but the new version needs to be uploaded on the server ASAP. Without an established, automatic process, the administrator must do it. If they're at work and have time, that's great, but if they're not or busy, we've got a problem.
DevOps removes these types of bottlenecks from the process. As a result, the managers have greater control over the project and the confidence that the continuous project flow is kept.
Similarly, suppose a PM or PO is heavily involved in the project and wants to be updated with all new releases. In that case, because of the DevOps infrastructure, they will immediately receive a Slack notification about the latest version. They will quickly download and click through it without waiting for the demo.
The entire process, from uploading the application to the easily accessible, clickable version, will happen quickly and automatically, giving the PM more control over the project and, ultimately, more peace of mind.