For most people a greenhouse or a polytunnel are a great bonus. They really help the grower but they're not necessary. Growing where with high winds at our 200m altitude and exposed position dropping the temperature, without a greenhouse and a tunnel what I could grow would be severely limited.
I wish I was one of those TV gardeners. Drop the seed in the hole, stand back and cut to a magnificent plant. Unfortunately I'm just human and things go wrong. Plans look great on paper and then the weather changes against us or life just gets in the way.