Answer:
1. It shows appreciation. Even if they didn`t do a deed or favor, giving someone a gift shows your appreciation for them just being in your life. Some people do it just to show it and don`t feel it, but it still satisfies the other person.
2. To help the less fortunate. There are some people who can`t afford what you have. It`s always nice to donate new and old goods to charities and give literally anything to people in need.
3. Spreading the joy. There might be something in your life that you find dear, either practically or emotionally satisfying to have. Sharing that object can help others feel the same joy! Like giving toys to children, or giving books to a library so someone else can read and enjoy it.
Let me know if you need more info!
Hope this helps!