Before we start any work....it need to be planned....who 
needs to do what,when and why
Test-plan:

similarly testplan provides you who, what, when and why ...

It provides you Test objective,scope, shedules, if 
automation tool required for conducting testing, what are 
features that need to be tested in these release,
when to stop testing, when to retart testing
Test deliverables, test methodology/stratergy,testing 
documents that need to be submitted, approvals stc.