Planting different crops together is useful for several reasons. Some help control pests, improve pollination, add nutrients, fill up some unused space or just provide some support to grow.
Runner Beans are nitrogen fixing so improve the soil, planting with Sweetcorn gives the bean plants something to grow through. Plant a few Sweet Peas with the Beans to attract pollinators
Marigolds and Nasturtiums attract pollinating insects, plant around the edges or in between crops like Courgettes, Nasturtiums will self seed in a mix of different colours plus the leaves and flowers are edible,
Onions & Leeks planted with Carrots help deter Carrot root fly and Spring Onions & Chives planted with Lettuce will help keep aphids away.
Radish and Lettuce both grow fast and are quick to harvest, sow in rows between other slower growing vegetables like carrots and beans early in the season.