Although you can sow broad beans, and most other beans, directly into the ground, if you start with pots you avoid having gaps in the rows.
See also: How to Grow Broad Beans
This is exactly how I like to grow my broad beans. Although they are hardy and won’t be hurt by low temperatures, the extremely wet conditions we have here in the wettest part of the country, means seeds are more likely to rot in the ground.
Like Sam in the video, I use 3″ pots except mine are square. As he pointed out, the roots migrate to the bottom of the pot so don’t worry if the potting compost breaks up at the top. Just pull some soil over.
The only point I can usefully add is that if late in getting the pot grown broad beans planted out, the roots will circle round and matt at the base of the pot. Just tease them a little so they will start into the ground as you plant out.