[naveen_p25]

Prasanna: Use seperate thread for your different questions, so that it will be beneficial for other members.

I am also advising you to please collect all your questions and post in one thread so that you will get answers in one go, it looks nice also.

Coming to your query -

We can't replace a particular step of manual test case into automation script.
Automation of the test case is required :

1. whenever the test case is required to execute number of times.
2. if the effort is more
3. much more calculations
4. more builds need to test and frequent changes in the system
5. resource availability and cost of the project

etc ...