Peas & beans produce a very long root as soon as they start growing. Ideally, this root should not be disturbed when the seedlings are transplanted into the veg garden, so that the root will continue to grow down to reach as much water and nutrient as possible.
If the seeds are started off in small pots, the root will curl round. When transplanted, the root will tend to grow in a tight ball. The root will not have access to as much water and food as it would if it were growing straight down. So the plant is compromised.
Therefore, if planting peas & beans in pots, use long pots. Supermarkets sell cream & yoghurt in narrow 250ml plastic pots. These are ideal, with a hole poked in the bottom. Long plastic pots can be found in garden centers or on the web, including special root trainers.