Growing onions from sets is far easier than raising them from seed. Plus, planting hardy varieties in autumn gives you a head start over spring-planted sets, as you can harvest the bulbs from June. Planting in autumn also means you’re using soil that would otherwise be bare, which helps to retain nutrietns that would otherwise be leached away by winter rains.
When planting sets, don’t force them into the soil as this can damage the base plate where the roots emerge. Use a fork to lift the soil slightly, if necessary.