You would bring her horses for the farm. You can't get horses yet so she is just there.
Sue tells you to try Prestige Ranch after you talk to her a couple of times.
So go to Prestige Ranch and buy horses for 50000 Krown from a man on the 1st floor of the main building. You also get to choose where to send the bill for the rest of 700000 Krown - Willheed, Lynevere or Naidhar. I chose Lynevere, cause it makes a big hole in enemies budget and, I suspect, may play a part later in the game.
After you get the horses, your expedition time gets halved, and Sue milks the steeds during the day. If you visit her at night she gives you buckets of... horse "milk". Each time you perform "stable wench" in castle tavern you need 1 bucket.
If you have captured monsters during any "Bounty" quests, you enable "Prison wench" in castle tavern.
Right now there is very little difference between 3 types of "wench" challenges in the castle (stable, tavern, prison). Looks like something to be developed in the future.