Probably everyone here knows all about the so-called deadweight loss of Christmas (people value gifts less than the givers spend, making the whole institution a huge waste). This article gives reasons why it's not as big a waste as it seems:

1. Ulterior motives. (I may have value for you having a gift, like if I give you a ticket to a show because I want your company.)

2. Signaling value. (There may be social value in proving our thoughtfulness.)

3. The ritual. (Traditions are nice.)

4. Extrospection. (In rare cases I may know your utility function better than you.)

5. Transactional Efficiency. (Sometimes it's easier for me to get you something than it is for you to get it yourself.)

6. Plausible deniability. (You may value something a lot but be unwilling to buy it yourself because it would seem too indulgent or something.)