Human Resources Mysteries: Why is HR Seen as a Useless and Inefficient Department?

This depends on how dedicated HR employees really are. If they are motivated and love their job, they will find a way to show people they are not useless and inefficient. If they are not motivated and don't really love their job, they normally avoid interacting too much with people and that's why their work is not understood. 

As I told you before, HR is not the best paid career ever, budgets are low, volume of work is huge; sometimes HR is seen as a parasite by management and that's why HR people are last when salaries, trainings and benefits are discussed. All this can get an HR employee really demotivated.