I have seen quite a few examples of creating associations between a one to many relationship, reflecting a typical association from a parent to child SQL Server relationship with a foreign key relationship.
What I find odd is that the examples seem to avoid creating the reverse association for the model.

In this example, we have Product and Category.
Why are we not implementing the reverse association to retrieve the Category given a particular Product?

It certainly can be accomplished using a bit of code.

How does not implementing the reverse association affect SharePoint functionality?

Thanks and looking forward to an answer to these questions!