Heterosis is one of the reasons to apply crossbreeding of breeds or lines. These effects of dominance are observed in all species and across species it can be concluded that estimates for heterosis are higher for characteristics with a low heritability and lower for traits with a high heritability. Heterosis is often substantial for fertility and health characteristics that cannot be easily improved by selective breeding due to the low heritability. Thus, improvement of health and fertility traits is often an important motivation to apply crossbreeding. In New Zealand the cross between Jersey and Holstein cattle has a long history and from 2000 onwards in North America and in Western Europe crossbreeding dairy cattle is increasingly applied. These crosses are applied to improve fertility and health characteristics of high producing dairy cows. These characteristics can hardly be improved in selection programs where milk production is an important breeding goal trait. Danish research show (summarized below) that in a cross of different dairy breeds heterosis can be utilized to improve these characteristics that have a strong relationship with longevity and overall profit.
Reference: Crossbreeding in dairy cattle: A Danish perpective. Sorenson, M.K. et al, 2008. Journal of Dairy Science, Vol 91 (11), pp. 4116-4128
The second reason for crossbreeding is to exploit the complementarity of breeds or lines: combination of the characteristics of two breeds or lines is favourable. An example is the cross of sows of a pig breed with a high litter size with a boar of a breed that gives a fast growth up to the slaughter weight. The cross leads to more pigs per litter that grow fast during fattening. This gives a higher profitability than keeping a purebred sow with the same litter size and a moderate growth of the piglets or keeping a sow of a breed with a moderate litter size and a fast growth of her piglets.
The third reason is that crossbreds combine characteristics that cannot easily be improved simultaneously in a single breed. An example is growth of lean meat and meat quality in pigs. These traits are within breeds or lines negatively correlated: animals with a higher growth of lean meat have a lower score for meat quality and the reverse: genes that influence growth of lean meat also influence meat quality, but they have an opposite effect.
The last reason for crossbreeding is the protection of the genetic improvement in the selection lines of commercial companies. They invest a lot in maintaining animals, recording of traits etc. By selling only crossbred animals to farmers, they avoid that competitors can use their purebred parent stock. And selling crossbreds to farmers, generation after generation generates the income for the breeding companies.