Jonas asked why he & The Giver had to hold the memories. What was his answer in The Giver?

The answer to this question can be found right at the
start of Chapter 14.  Briefly, what the Giver says is that he and Jonas have to hold the
memories (even if they are painful) because the memories give them wisdom.  This wisdom
can be used to help the Community when that becomes
necessary.

For example, the Giver says, the wisdom that he
has gained from the memories tell him that overpopulation is bad.  He used that wisdom
to help the Community some time previously when the Elders were thinking about whether
to have each birth mother have more kids.

So the Giver and
Jonas are supposed to be the wisdom of the Community, remebering things that might be of
help at some point.