Code is fine in a trusted environment, however sometimes you need to give another process/user/actor a limited environment. You could give them a limited run time environment, however that is often difficult
For example, consider you have a stream of events that you need the user to be able to filter server side, such as all events that have a field of "name":"bob".
I will agree with you that these systems often increase in complexity until you have finally built a small language runtime. It can be difficult to distinguish where the descriptive language ends (eg binary operations) and the other begins ends and the other begins (eg sliding window, custom algorithms).
For example, consider you have a stream of events that you need the user to be able to filter server side, such as all events that have a field of "name":"bob".
I will agree with you that these systems often increase in complexity until you have finally built a small language runtime. It can be difficult to distinguish where the descriptive language ends (eg binary operations) and the other begins ends and the other begins (eg sliding window, custom algorithms).