The Women's Land Army was first created during World War One in the UK. This was an era when a great deal of farm work was done by men. With so many young men called up for the armed services, there was a real gap in farm workers. Hence, the government called on women to fill this gap. The same situation arose in World War Two - home grown food was needed and the men were not there to harvest it. Hence why the government resurrected the WLA.
For more information, see this article.
World War I books