Why are there so many ways to following the steps of scientific methods? Why do we have to do extra work to find a perfect right answer?

Because there is more than one way to skin a cat.

I would say it is because there trying to avoid cheating.

In our day and age, it is easy to look up an answer.

So yes on nearly all questions asked you have to give the formula and work through to how you came about the answer.

"Why do we have to do extra work to find a perfect right answer"? So you don't put too much U235 together, reach critical mass and make a fission reaction!

It is to eliminate possible sources of errors. Most experiments using the scientific method go to great lengths to eliminate possible sources of contamination and introduced unknowns. This allows the person who is doing the experiment to know that the results is because of they're studying and not because something else. It also helps narrow the context and make the results mean just a specific thing and not be interpreted as something completely different. This has happened.

The peer review helps to ensure that the information withstands the rigors of the scientific method, if they don't then papers that link Autism to Vaccines will be considered valid.

It is to make sure that you can say something with out a doubt.