When you update a Kubernetes deployment in the API directly you're essentially modifying a global variable. If you do this as the result of some imperative process (e.g. a CI pipeline), you don't have any record telling you what the value should be - you only know what it currently is.

You're also only modifying a single piece of your whole state at any given time, meaning that if you have the services A and B and their deployment pipelines independently modify state - you have nothing that declares any relationship between which versions of these should be deployed together.

There's a little piece of infrastructure wisdom I've learned over the years, I refer to it sometimes as "tazjin's law":

Any infrastructure component not controlled by a reconciliation process will eventually fail.

Versions of dependent components will get out of sync, configuration is being deployed independently of the application, and so on.

In order to reconcile your current state with your desired state you must know what your desired state is.

Does that explain it?