Purpose of voting: I try to use all my votes partly because I tend to feel that voting is a "civic duty", but mostly because it forces me to read a lot of questions/replies, and I learn a lot in the process. That said, I usually upvote anything that seems like an interesting topic or decent answer to a question (and also often posts/replies that include some humor especially if it is relevant to the topic in some way.) I guess I tend to read philosophical discussions or short technical discussions much more than long complicated ones just because of the time involved in understanding the topic.
chas