To release bug or defect free software tester plays a vital
role in Software development life cycle

I am not satisfied with your answer. Can you please explain
me briefly?

Because tester will understand total BRS and CRS and they 
will conduct all possible tests to capture the bugs in the 
application.For this they will do both positive and 
negetive testing to verify and validate the 
application.Finally they release almost bugfree software 
for end users.

Let us see the testing cycle how docs will be reached to 
Teting Team

Business Analyst->Change Request or Business Requirement
SSG or Architecture Team->SRS
Developers->various Design Docs w.r.t to their Systems and 
unit test reports
Integration Testing->Integration Test Report
System Acceptance Testing->SAT Report
User Acceptance Testing->UAT Report(Almost equal to live)
and finally application will move to production

So all the documents are bundled and will be given for the 
testing team.Tester should read perfectly and should 
prepare test plan.And that should be accepted by all 
stakeholders like dev,SSG and BA and also UAT people.

Once test cases are finalised testing can be started.If any 
defect is missed, sometimes loss will be high for the 
organization especially in case of mediation and Billing

Thats why I mentioned bugfree software.Hope u understood 
the importance.

thank you very much

The word Quality it self identify a special thing for 
others. In software industries Quality is more important 
for providing a bug free product. and Testing is a 
profestional job and testors  those who know this techniqu 
how to do it.

To provide bug free product

If you are already a tester / QA  you should now this basic
thing right? 

Any how let me clarify it.

Tester / QA is appointed to maintain the quality of the
product. For this QA should know the entire end -to- end of
the developing project in hand, and when they try to keep
the quality of the product they come across some kind of
disturbance which is nothing but we terminate as Bug,
Developers resolve it and fix the bug by fixing it QA does
re-testing and regression testing appropriately and thus
they keep track of all the Bug which come across of blocking
the quality of product. So in this way QA makes a major step
in maintain the quality of the product. 
Quality of product stands with different issues 
1. Usability issues
2. Performance and storage issue.
3. Functionality issue
4. COmpactibility issues too.

These are some of the part of quality of the product  which
test engineer does which is divided into different levels
which we call it as STLC ( Software Testing Life Cycle).

Hope this might clarify your doubts.

I feel that tester plays their complete time for the quality
software when compared to others (like developers,managers)
who are playing their roles in the software life cycle.

They will identify each and small bug/defect present in the
product and they will make others to fix them.

So the testers play a important role for good quality of teh
product.