For me, it's mainly to express my gratitude for the subscription. I've gotten very nice replies to my thanking, saying that they enjoyed the series and what-not. I think it's a great, personalized way to interact one-on-one with your readers.
Even after I got Staff Picks and Community Picks, I still keep up the habit of thanking.
I find it easy and fast to do, so it's fine for me.
From a marketing angle, it also shows the other series that you have (if said reader didn't see it)!
Also, as a creator, I like to see other thank-yous on the wall and like what Charlotte said above, I do find new things to subscribe to.
The only reason why I mark my library as private is because people irl have gotten confused as to which are the stories I've actually written!
My parents, for one, thought that the novels in my Library were mine!