Seeds from a good catalog-sales company are basically guaranteed to be a) fresh and b) properly stored.
OTOH seeds you get at the local big box store, it is wise to squint at the 'packed for the 200<whatever> season' print to make sure they;re not last year's leftovers, and you have absolutely no way of knowing how they've been treated after they left the packaging company. Poor storage (too hot, too cold, too humid, whatever) can really mess up viability.
Plus, there are a lot of things you just can't GET at the local big box store (or farm store, or garden center, or whatever).
Not to say you *shouldn't* buy from your local store, I'm just laying out the issues involved.
Pat