@katart17,
You don't have to add tracking details in Subscribe2, it's there as an option if you want to use it.
By adding tracking data in that field you'll be able to see how many visits to your site originate from FeedBurner and how many originate from Subscribe2. If you are not going to look at these stats or don't care where your visitors are coming from then it a pointless exercise adding tracking in the first place.
