To make certain a thing will hold. Ensuring adds a guarantee where there was only expectation.
Anglo-French enseurer, en- plus seur (sure), from Latin securus (free from care, safe), se- (without) plus cura (care).
Its root is freedom from worry, not merely certainty: securus named a state without care, the same root behind secure. To ensure something is to take the worry off it, a different gift than simply predicting it will hold.