Gather local vs process rules

Can anyone explain why you'd need to process rules in a task sequence gather step?

As far as I can see, you'd only need it if you want to set a variable to a value that was not initially available before TS start.

I have a CustomSettings.ini which has per-TS settings that are evaluated by the LTI wizard before TS start. The wizard allows you to select the TS, then a modified VB script re-evaluates the variables so that per-TS settings are applied.

I'm assuming that these variable settings are propagated along during the TS, so there's no need to re-evaluate the rules after they're initially set.

Am I understanding this correctly?