To do any task we need a plan and without planning we cant 
workout the things in a order.

To test a entire project we need a some planning here test 
plan comes into the picture n you can have some better plan 
to test the application like what kind of testing should be 
good to test this project, how many resources are required, 
time allocation and what kind of tools are required and 
what the deadlines for this project. And what is the scope 
and what approaches should we follow n what kind of 
hardware and software required for this project so 
everthing will be planned here.

sorry i dont have time now so i just in few sentense, so if 
you have doubts contact me at my mail

Test Plan Gives an idea about what you are testing and what 
is the objective of testing a scenario/feature/build
Test plan gives a complete details to the test engineer who 
is executing the test cases. It gives why we need to test 
(i.e. the Objective of testing) the tools/ or the setup 
needed to execute the test case, it also tells the test 
engineer like by when the test case need to be get 
completed giving the start date and end date for testing.It 
also gives the results like if a test case works fine what 
should be the test engineer should do like 
Pass/Fail/Block/Not executable.

A better question is why plan anything? You plan those 
things that you know are more complicated than what you can 
do by the seat of your pants. You plan things for which 
order and completeness are important. Even the simplest 
software system is so complex and is so prone to failure, 
that planning to test is just as essential as planning your 
design.
The second major reason is that, as an industry, we are 
doing a lousy job at releasing quality products. Current 
industry standards are that tested and released software 
averages more than ten significant bugs per 1000 lines of 
code. How many hundreds of thousand of lines was that last 
project? Thinking ahead and planning your testing is one 
way to cut that down.
All of these are good starting reasons for having a 
software test plan.