When you buy seedlings from a nursery you’re paying professional growers for their expertise, the overhead of their facility, and of course the containers, potting mix, and seed – as well as the retailer’s markup. Generally speaking it’s a good deal.
The downside is that you’re stuck with the varieties they select, and if you’re the type of person who enjoys perusing seed catalogs, or growing specialty plants then you probably want to grow from seed.