My use case: I use pipeline stages to run delivery of projects as the customer journeys through the process. The pipeline stage informs me how much in cash / revenue is stuck in a pipeline stage currently, but as I take 50% upfront the value in that pipeline stage is always overstated.
So in this case the automation would update the Opportunity Value by reducing it by 50% but only as it enters the pipeline stage.
There are many other use cases others could bring but to simplify it:
{{FIELD}} is updated by a {{FORMULA}} (i.e. set as a value, or a calculation).