Why do external auditors evaluate and give their opinion as to the effectiveness of internal controls in performance of audit?

The external auditor must to take into his consideration the examination carried out by the internal auditor so that this examination be done to complement and not a substitute him first and he cares about is the external auditor, accounting controls that need to be hosted.

In addition to the study and evaluation of internal control enables the auditor to make decisions about the possibility of unreliable or expand the scope of the tests 

And if it appears to the external auditor ineffectiveness of the existing internal control system must be performed to provide suggestions as it deems appropriate, 

Therefore, examination of internal control in the establishment is the duty of the external auditor.

Internal Audit is not compulsory but optional on the part of the management.  The effectiveness of the internal Audit reflects in the nature of disqualifications by the external audit reports.  External Auditors point out areas of weekness in the internal system which is to be strengthned and non-covered areas highlightned so that Management can modify internal control system with improved parameters for internal audit for effectiveness in the Control.

Periodic or specific purpose (ad hoc) audit conducted by external (independent) qualified accountant(s). Its objective is to determine, among other things, whether (1) the accounting records are accurate and complete, (2) prepared in accordance with the provisions of GAAP, and (3) the statements prepared from the accounts present fairly the organization's financial position, and the results of its financial operations. See also internal audit.