That's when I spent some SBD to promote the posts, which then appear under the "promoted" link on your blog. I'm not sure if it helps to promote one's posts, but I usually spend a tiny bit of SBD for each post just to ensure that I get more exposure in the "promoted" stream.
As for the "NULL" recipient, I read somewhere that all payments for such post-promotion are not actually given or sent to anyone. Those payments are just recycled back into the overall pool of SBD. Therefore, it says "Null."
Hope this clarifies.
Will it help in building reputation?
No, not really. Promoting a post gets it more exposure, but it may not help build reputation, and it might not even help one earn more Steem or SP. But I do it for exposure and for long-term, future success.
Oh yes, it answers my question well. Thanks for sparing time for me and giving me such detailed answer