Test Cases are needed while performing Mannual/Automation 
testing. 
Yes, It needs to be documented in a proper and in a more 
clear way sothat any one should understand approach and 
steps to execute the requirement. Again the documentation 
needed according to the company. 

Advantages:
1.Gives an approach, Decription, Pre-conditions to achieve 
the exepect result.
2. Test Cases are useful while writing Tracebility Matrix.
3. It shows or avail the traciblity of the entire 
requirements. 

Disadvantages:
1. Without or miswritten Test Cases or human errors in the 
Test Cases cause the project failure.

Test cases are to be documented.
Adv: 
1)Keep record of the test performed on application so that 
it can be reused.
2) Review of test cases to check for the correct testing of 
application based on the input data, conditions, actual 
result of test.
3) Gives an idea of how much testing is covered and time 
required to test the remanining functionalities.
4) If out of time then can pick the major functionality 
test cases to perform testing at faster phase.

disadv: I guess i am not finding one.

Let me know if this is correct.