My understanding is that seed beds allow you to manage your land more efficiently, but not necessarily produce better veg than if you plant out the seedlings direct!
For example, if you are growing winter brassicas, they need to be sown by summer, and will soon get too big for seed trays, but if you plant them straight out they will be taking up that bit of land for ages as they slowly grow. If you set aside a smaller piece of land for a seed bed, where you can plant the brassica much more densely, you can grow something fab and summery like beans or tomatoes on the other bit. When they are done you quickly clear the land and transplant the brassica at their final spacings.
Or something like that?! Well hopefully that's what I'll try this year anyway