I normally try to thank every fav and watch as it does mean something to me.
For others I dont know.

It's all advertising.

i just want to hug you and look at art i can never draw

Well, that is very nice of you.

(Anyone can draw. It just takes a lot of work.)

I usually don't respond much to favorites anymore or even watches but if that person comments then I'll certainly respond because they took the time to tell me what they thought. I think I'll actually be adding you to the watchlist, I'd love to see more of your sfw pieces although I do think your rhino lady is pretty cute. I appreciate your comment and hope you may return to my gallery from time to time for more monsters because there will certainly be plenty of that.

Well, both. I always write a thanks-message. I include the person's name. And if I like their work I'll watch them back. Additionally, I do it to advertise. This is also why I've tried to make my icon eye-catchy and representative of what I do.

That concept does pass through the mind of many, including myself.
But, i'll bet a lot of people genuinely do that purely for advertizing. especially, if they focus on other very popular artists in particular.
I try to have a legit reason to comment or thank. like if i like someone's taste in Favs, or if they need a good word of encouragement.