The first few months of an infant's life are spent lying down. Looking up at the world, or down on the floor during tummy time. So when a baby learns to sit, even assisted at first, she gets an exciting, new perspective.
My babies spent some time, before they were able to sit on their own, propped up against pillows or in a special chair or surrounded by cushions. It's one of those tasks that you do as a parent that is a lot of work and not a lot of reward. Your baby won't sit very well, in the beginning, and you may even let a little sigh out when she slowly topples over to the side and back into a horizontal position. But you keep trying, because sooner or later, your baby will sit up, all by herself! Until then, you watch her like a hawk and don't leave her side because you know it's only a temporary position.
One day, though, when you set her up on her bottom and she looks around at the room, she'll stay there. Just for a few seconds at first, and still probably surrounded by pillows (just in case) but it'll happen nonetheless. And you'll lay her down and go grab your camera so you can show all of your friends and relatives what your baby has learned to do, and hope that you can capture a clear picture (as opposed to a blur as she goes down again).
She will be just as excited, too, just as she is anytime she discovers that she can do something new. She will be proud of herself as you clap and smile and cheer for her. And it is another one of those moments that makes the long hours and late nights so very worthwhile.