In the case of Avatar, it was a bit of both. Most episodes did have a satisfying conclusion, but it was nothing like the Simpsons or X-files. The show was a serial and it had a story arc that played out over three seasons with each season having a theme. It had been planned out that way very early.
I love shows like that where the creator has a beginning, middle, and ending in mind from that start and so it can't just be stretched out forever until they've run out of ideas or the audience gets bored and ratings drop off. It's just better storytelling to have a compete story. In TV animation that sort of storytelling isn't as common here as it is in anime which helped Avatar stand out, especially compared to what other shows were airing on the channel at the time