When you get the dinner pacakge, you get a name badge that qualifieds you to receive 15% off of all Epcoet purchases all day.
If you are planning on buying some Epcot merchandise, then wait for your CP day and get it discounted.
Personally, we have always chosen the package to eliminate the waiting in line. If it is cold or rainy, the CP dinner package guests get a change to sit under the front cover.