2 and 3 year olds are just learning about sitting in a group setting and staying in their spot.
If you give them a visual place to sit or stand, it helps them stay in that space.
We found these little mats at Target. They are about $3 each and are just the perfect size for preschoolers. When it’s time to start Large Group, we play the clean up song. Preschoolers help to clean up all the toys, and then they go get a mat. They put their mat on the floor and during Large Group time we get them to sing songs & stay on their mat. During story time, they sit on their mat. They often need to be reminded to “stay on their mat” but at least it gives them a defined space where they need to stay.