The Jarrow March took place in October of 1936 because of the economic hardships caused in that part of England by the Great Depression. The people who took part in the march were protesting the lack of economic opportunities in Jarrow and were trying to get the government to create jobs in the city to alleviate their problems.
During the Great Depression, Britain in general suffered economically, but places like Jarrow that relied on heavy industry were hit the hardest. In 1935, a major shipyard in Jarrow had shut down. This contributed to the fact that the unemployment rate in the Jarrow area was as high as 70%. Additionally, the government did not offer as much in the way of unemployment insurance as it does today. The despair of the people of Jarrow over their economic conditions led to the march.
The link below says that the march had a minimal effect. The government was not willing to pay much attention to the marchers. When their petition was presented to Parliament, it was accepted but no proposals were made to help Jarrow. The Prime Minister refused to see representatives of the marchers. However, two major industrial works (ship breaking and steel) were established in Jarrow by 1939. It is not clear whether the march helped cause this to happen.
The Jarrow March happened because of the economic hardships caused by the Great Depression. It did not achieve anything in the short term.