Sheep, like many other animals, sit down to rest, eat, or graze. It's a natural and necessary behavior for them.
There's a bit of a misconception about sheep and sitting down. It's true that they are prone to a condition called "grass tetany," which can cause muscle weakness and tremors, sometimes making them appear as if they are sitting down when they are actually collapsing. However, this is a medical condition caused by a lack of magnesium, not the act of sitting down itself.