I hit reply and leave a comment as my way of saying thank you for visiting my blog and some upvote and resteem. Many leave quality comments of photographs which enhances my blog. It can also create a dialog where we can get to know each other better.

I resteem some because of their quality content. For newcomers it will get their post out to many more people to view and maybe they will follow.

Hope this answers your question.

I upvote each post (most) because it is a way for them and me to earn a little steem and SBD.