Yesterday, I presented a class entitled, "Which items do not need kosher certification?"
The fundamental point is that kosher supervision is required for the following seven reasons:
1. To ensure that ingredients are kosher (including ensuring that bugs are not present);
2. To ensure that equipment is not used in any way that might compromise its kosher use;
3. To ensure that products are properly sealed, and that there will be no confusion between kosher and non-kosher product en route to the consumer;
4. To ensure that the food is prepared by the people who are supposed to prepare it, averting bishul akum, pat akum and stam yeinam problems;
5. To ensure that potential halachic problems are detected and properly resolved;
6. To ensure that meat, dairy and pareve are all kept separate;
7. To ensure that Shabbat and Yom Tov issues are properly handled, particularly in establishments with Jewish owners or employees.
If all seven of these points can be addressed without supervisory presence, then certification is not necessary.
Rabbi Eliezer Eidlitz has an interesting list of no-supervision-necessary products here. Not everyone will agree with his list, but I applaud his bravery in publishing a list at all! That is truly a task for broad shoulders.