If you have a reservation for Tusker House between 1:00 pm and 1:40 pm, you can get a priority ticket for the 3:15 pm Finding Nemo show. It's great because you don't have to wait in line to see Finding Nemo. The downside can be the timing. If you have a late ADR and they're running late, you might find yourself having to eat very fast because Finding Nemo is a bit of distance from Tusker House.
We did it once and it was good. We had a 1:05 pm ADR, but were seated late. We did finish lunch without being to rushed and then headed over to see the show. We bypassed the line and got great seats. There is a priority seating area, so you'll sit in that section (it was to the right near the front when we went).
If you want to see Finding Nemo, having the lunch package is a way of knowing that you will get to see it without having to wait in line. However, if you don't like Tusker House or are not normally into large lunches, then it might not be worth it.