The major economic issue during the great depression of the 1930s that concerned john maynard keynes was:

Keynes was concerned with government influence in times of a recession.

John Keynes was a British economist who focused on trying to understand the Great Depression during the 1930's. While studying these causes, he developed a concept known today as Keynesian economics.

In this theory, Keynes argues that there is bound to be prosperous and awful times in a free market society. Depending on what the economy is doing, government should adjust accordingly. For example, during a time of great economic success and prosperity, the government should scale back their influence/spending in society. However, when the economy is struggling, the government should increase spending and lower taxes in order to help the American citizens.

Very useful. it is to learn because history repeats itself so learning about it those mistakes not happen again.