UseStateForUnknown(): Copies the prior state value, if not null. This is useful for reducing (known after apply) plan outputs for computed attributes which are known to not change over time.

Why is this modifier needed?

Essentially to simplify the terraform plan output. The interval_min attribute will only change its value when modified by the user.
As a counter example, alert configuration has an updated attribute that is computed and is not set with this modifier. In this case is seems appropriate to have the message (known after apply) shown on the terraform plan because on every apply it will be modified.

I was reading this plan-modification#plan-modification-process

Plan Modification Process
When the provider receives a request to generate the plan for a resource change via the framework, the following occurs:

Set any attributes with a null configuration value to the default value.
If the plan differs from the current resource state, the framework marks computed attributes that are null in the configuration as unknown in the plan. This is intended to prevent unexpected Terraform errors. Providers can later enter any values that may be known.
Run attribute plan modifiers.
Run resource plan modifiers.

I am wondering if currState.IntervalMin.isNill is false but currState.IntervalMin.IsUnknown() is true when calling terraform plan. Could you verify? if that is the case we should add currState.IntervalMin.IsUnknown() in all of this ifs

Yes your statement is correct, I could verify that the attribute in currState comes as unknown as it is an undefined computed attribute.

In this case, since it is a value that is always defined in the api request/response (defined as an int in manual sdk), we can simply define the value in the model. This is what solves the inconsistency issue when doing the import operation, because when importing you have no state/config to verify if field was defined by the user, so value coming from the api is set directly.