Landrace plants are an insurance policy. They are plants that are adapted to the environment in your garden and are resistant to the common diseases there and the common pests there. A notorious case of mono-cropping, or using the same cultivar all over, was the Irish potato blight that led to starvation or emigration from Ireland by so many individuals. Everyone had the same cultivar of potatoes and they all got the blight at the same time.
In contrast, landrace plants contain a wide variety of genetics and are able to adapt to diseases and pests. These plants are open pollinated. The only down side to landrace plants is that it takes many generations to achieve seeds that truly do well and are resistant to the local problems in your gardens.
Growing a landrace crop is relatively easy. Instead of separating the different cultivars of your plants by as wide a space as possible, you plant all of the cultivars you can get of that particular plant together and encourage cross pollination. For example, you would plant all the cultivars of green beans, including mixing climbing and bush green beans. When the plants have vegetables, save some from the best plants –the ones that produce well, have bountiful harvests, and taste the best.
The next year when you garden, plant the saved seeds alongside the commercial seeds. Again, save seeds from the best plants you have. Do this for three or four years. These seeds should contain a greater genetic variability than any you can buy. This genetic variability allows them to adapt to local conditions and pests better than store bought plants.
Each year, when you grow your garden, you need to keep records on what plants did the best. Those plants are the plants you want to save the seeds from for planting next year. Gradually, you develop a line of beans that is ideal for your location. They grow well with minimal attention, resist pests and disease, and bear well.
There are a number of books that discuss developing landrace plants from seeds you have saved from the previous year.
SEED BY SEED by Suzanne Ashworth is very good. Some seeds require special handling to dry properly so they will keep for next year. She tells you exactly what to do for each seed so that it remains viable for the next year.
BREED YOUR OWN VEGETABLE VARIETIES by Carol Deppe explains the genetics of breeding your own landrace plants in a way that anyone can understand. She tells you how to mix different cultivars of the same vegetable to get a hardy plant that produces genetically diffuse vegetables. She also discusses problems with drift in the vegetables and with too much inbreeding. The remedy to that is making wide crosses and cross breeding your vegetables with their wild relatives to make sure the plants maintain a wide variety of genes.
Done well, developing landrace plants and planting them every year will make your plants hardier and more resilient if there is some kind of wide spread disease or pest problem.