I'd like to say what the others have said here are possible; maybe it's all about saying hi, to show us that they're watching us, they still care about you, or there're things they want you to do.
There's one other thing however, came across my mind upon reading your post.
She didn't do much with us except sit down on the chair and have short discussions. Is there some message that's trying to reach me here? Cause every time I dream about her she doesn't talk much or has anything to tell me. In fact when she is discussing something I can't see to who she is talking to.
This descriptions.
These lines of yours were actually made me thinking that most of us believers would believed that passed away people would still have the same mundane traits like the living ones. If so, please look at your lines again, what can you see there now? She sat on the chair alone, and speaking to somebody you couldn't see.
What does it mean for us, living human, when we see one of us is showing that kind of attitude? It could be a form of deep/heavy loneliness. Or sorrow, depends on the situation at hands, I think.
Maybe, your late grandmother missed you, and you saw her talking to 'you' in your dreams-but you saw it from a third person perspective.
If that's the case, you might want to try to offer her a prayer or two (burning some incense for her is also a good idea), and it'd be enough to make her-at least, rest in peace, because you still remember her. It'd be better if you can also come to her grave in person yourself. Try it, and see if the dreams would stop-for a time being. If the dream happens again in the future, do the exact same thing again, since like in the real life where close relatives are always missing each other over time, so are they.
I've had almost similar dreams as well, but not as often as yours, maybe once in a year or two (since 2010, when my grandmother passed away). In my case, it was my maternal grandmother, and in my dreams she displayed a pretty much similar attitude you've described in your dreams. Praying for her is what I did after having such a dream and she'd stop appearing in my dreams for a moment.
But as I said, it'd be depended on the situation at hands, as in different situations, she might have came in my dreams more often than your grandmother in your dreams.
Asian believers, are known to have fear about having some certain dreams because those kind of dreams are believed to be a bad omen (I think I've explained one of the worse dream I'd hate to have on one of the threads here, if memory serves), so do I.
In a number of occassions, my late grandmother came to wake me up everytime I've almost had one of the bad dreams-mostly by knocking hard on my room door in my dreams.
Well, I guess that's one of her ways to take care of me, and also a form of her gratitude for my prayers to her.