There great for when your little one is pushing forward wanting to sit up but can't actually do it yet, they support their back. Charlie was always trying to sit forward in his bouncer and it just didn't look safe so we got one of these and it's like his little armchair. We got ours from Mothercare:
http://www.mothercare.com/invt/lt1932&bklist=
http://www.mothercare.com/invt/lt1932&bklist=