Why is the lawmaking function of Congress central to democracy?

The answer is: because it is the means by which public will becomes public policy.

Members of the congress are elected by the citizens in order to carried out their aspiration to the lawmaking process.

The citizens cannot force their will to the government and directly vote for the law that must be passed by the government. BUT, they can elect a congress who has similar goals, value, principles as them and trust that congress to make the best possible decision for them.

Because of this, we can say that the lawmaking function of congress is an important means to make public will to become public policy.