The rise of the middle class improved the economy in the time of the Crusades (also known as the “High Middle Ages”) because it was people of the middle class who allowed a boom in trade to occur during this time.
Earlier in the Middle Ages, people had only been able to engage in subsistence farming. There was not enough surplus food to support a middle class. Around 1000 AD, this changed. By the time of the Crusades, there were food surpluses and so a middle class could arise. This middle class typically worked as artisans or as merchants. The artisans made various kinds of goods and the merchants traded them around Europe. This trade helped the economy to grow. It enriched the people who engaged in it. As they became richer, they were able to buy more things both from their local surroundings but also from more distant places.
In this way, the rise of the middle class was both a cause and an effect of the economic improvements. As the middle class grew, so did trade. As trade grew, so did the middle class.