Can Internal Auditor add value for the company through his work? If yes how?

Not he can? he has to do it as per the definition of internal auditing:

Internal auditing is an independent, objective assurance and consulting activity designed to add value and improve an organization's operations. It helps an organization accomplish its objectives by bringing a systematic, disciplined approach to evaluate and improve the effectiveness of risk management, control, and governance processes.

So if he don't add value he is not doing internal auditing.

Assurance, Insight, and Objectivity, simple three words explain how Internal Audit value

yes can do so, just by improving/monitoring the  performance of departmnets 

Yes he can. The key objective of Internal Audit is to add value to an organization.

Internal Auditor provides evaluation of internal control, risk management, and corporate governance.