Does ISO Certification have impact on a service delivery or is it just a requirement?

ISO has good impact on service delivery. Particularly it benefits in cost saving, customer satisfaction, access to new markets, increased market share , environmental benefits. 

I don't have any personal experience with ISO but from what I have heard, it has impact on service delivery.

1. Increased customer satisfaction

2. Improved internal communication

3. Reduction of product-liability risk

it should be service 

 has  impact on  service  .Concerned   the  availability  of  quality  service  or  product  And  get the trust  of  customers

I fully agree with the answer been added by  Mukesh Varadhan.

The main intent of ISO's and Other certificates as such, is to organize the way various jobs and task should be carried out /performed. IT also help to mainstream the work flow in any company or organization.  as a result you maintain the quality of your services and/or products delivery.

ISO certification is a statement to your stakeholders, employees and senior management that the business wishes to operate to a set framework in order to achieve its company objectives. Be this customer satisfaction objectives, production objectives or environmental objectives etc. By setting yourselves the task of maintaining an external certification you are proving the company’s commitment to these objectives as well as increasing the credibility and customer confidence in the brand / service or product. For example by running a quality management system, a company can stay in control of its processes and procedures, ensure if anything does go wrong it is rectified quickly, efficiently and to the satisfaction of the customer. It can also ensure a smooth line of communication between employees, suppliers and customers at all times.

Few of the certification for providing customer satisfactions are: 

ISO9000 is a quality management standard that presents guidelines intended to increase business efficiency and customer satisfaction.

ISO/IEC20000 is the first international standard for IT service management.

ISO14000 is a series of environmental management standards developed and published by the International Organization for Standardization ( ISO ) for organizations.

ISO/IEC27000 is part of a growing family of ISO/IEC Information Security Management Systems (ISMS) standards, the 'ISO/IEC27000 series'

etc....

All these are a framework which provide the best and ensure the customer a trust worthiness and increase the satisfaction level.

Its not a mandatory certificate, but it will help to create a trust in the mind of customers. Also some companies will share data between the ISO27000 certifies companies only.

article :http://www.isoqsltd.com/faq/#3:_What_is_ISO_Certification?