In The Giver, what purpose does a comfort object serve for society?
While the comfort object appears to serve a more significant purpose for an individual than it does for society as a whole, let's look at how it serves purposes for each.
First and foremost, it is used to comfort individual infants. In our society we would call the comfort object something like, "his favorite teddy bear." In their community, it is less personalized by name and is also taken away from the child at a predetermined age, perhaps because they view it is a crutch preventing further growth at an older age. So, the purpose it serves for an individual is simply to soothe an infant.
For society, the argument could be made that it serves the purpose of providing a measurable level of affection and comfort. In this community it appears that being able to quantify things is important, so this comfort object could be seen as just one more item on a checklist of "good nurturing." If a child did not bond with the comfort object, I could also see where this particular community might use that as a reason to consider the child abnormal or lacking in development. In that case, the comfort object might also be an evaluative tool for nurturers and parents.