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Service accounts in Kubernetes allow you to enforce RBAC for all Kubernetes resources in your cluster. Service connections in Azure Devops allow you to use RBAC policies for infrastructure, including Kubernetes clusters.
Asserting RBAC on all systems that have an associated cost to operate is a great start to keeping costs under control.
Service accounts are neat, they allow processes impersonate a user and do things. Kinda like a computer/av system in a conference room. ie: you send a meeting request to an email account to include the conference in your meeting. It has the side effect of keeping people out of the room, but it needs a reference in the system so that it can be addressed accurately. Anyways... let's say we want to accurately address a computer capable of sending 10000000 pods to the cluster. That might get expensive if it can do it willy-nilly and anything with costs associated shouldn't be willy-nilly.