So that I have them "all" to look at and decide. Sometimes I'm talking to someone and the 4th out of 8 comments is there best one. Also I review the whole post, proofread again, and follow the commenters I wasn't following.
And it reminds people of me. They commented a few days ago and now they are being upvoted. So sometimes they come back right away and keep commenting or go to one of my other posts.