Companion planting for cabbages is about providing protection from pests and providing nutrients.
Because they are very heavy feeders (think about those huge glorious big leaves) cabbages can be difficult to grow. They are also a favourite food of white moths. Seedlings can be stripped and destroyed overnight by white moths. Companion planting can help, although not completely eradicate, with this problems.
Companion Planting for Nutrients
Big leaved plants like cabbage (and other brassicas like cauliflower and brocolli) need lots of nitrogen. Lots. Of. Nitrogen. By companion planting with legumes and other nitrogen fixers like beans and broadbeans, you are placing back into the soil a vital nutrient for cabbages.
Companion Planting for Pests
White moth can be an absolute disaster for brassicas, especially when they are younger seedlings. They can strip mine a cabbage overnight in warm wet conditions. At some stage in your gardening, you will loose an entire crop of cauliflower or brocolli or cabbage to the white moth caterpillar. There is no one sure fire way to prevent white moth infestation, so you need to use a combination of approaches. Companion planting is one of these. My interplanting with the plants listed below, you are confusing the white moth by mixing up the visuals as well as the olifactory. The white moth can’t see or smell the cabbages properly.