1. To amplify or spread tweets to new audiences.
I don't do this as much as #2, but I do like to help spread good information I think is relevant to followers.
2. To entertain or inform a specific audience, or as an act of curation.
Curator should probably be my middle name. That is the second biggest reason I am on Twitter. The first? To grab all the new, good info I can find to get better at what I do. I am addicted to good information.
3. To comment on someone's tweet by retweeting and adding new content, often to begin a conversation.
I think this is directly related to #4 and I engage in this practice as well.
4. To make one's presence as a listener visible.
If I were making a personal observation here, I would add to make my presence as a "trusted" listener visible. How do you establish that? By retweeting info that helps make other people better.
5. To publicly agree with someone.
I'm not real political so I don't do this very often but find myself doing it with certain people like @AmberCadabra of Radian 6 who engages in some personal funny stuff once in a while. @LizStrauss is another one who brings this out in me. Twitter can't be all about business.
6. To validate others' thoughts.
There are smart people out there saying things and finding things much better than I can.
7. As an act of friendship, loyalty, or homage by drawing attention, sometimes via a retweet request.
Yes, yes and yes.
8. To recognize or refer to less popular people or less visible content.
I learned to do this from my daughter. She likes to draw attention to people that are not well known but know well. She is also good at inviting people to guest blog for her that have never written a blog before. That is a great way to lift people up and work at being a trust agent. But this needs to be genuine, not convoluted. @lizstrauss does this well.
9. For self-gain, either to gain followers or reciprocity from more visible participants.
Yes, I am probably ashamed to admit I do this. But, I will admit that I like to retweet engaging people. I'm not trying to build up my number of followers, just the quality.
10. To save tweets for future personal access
To find a tweet later by searching myself? Hmmm--have to think that one through. Sounds a little odd to me.
Why do you retweet? Do you see yourself in one or more of the "whys" above?