There is no way to really say adequate thanks, but a memorial gift makes a stab at trying to do so. Secondly, if the gift is something that the deceased person had previously seen as a need, there is satisfaction in carrying out their wishes.
Finally, we survivors find it hard to accept the fact that after a very brief time, that person who was so important to us will no longer be of any importance to anyone else. It is our feeble attempt to gain a smattering of immortality for them.
At the risk of being thought a bit unstable, I confess that I can imagine the departed loved one peering over the bannisters of Heaven and giving us a grin and an “Atta’boy!” for our efforts in their behalf.
I suppose a student of psychology would primly tell me that a memorial gift enables the survivors to find a measure of closure to emotional anguish, but I like my daydream of a grin and a high-five better.