Question

Why do experiments need to be repeatable?

Answer 1

If they were not repeatable people would think the experiment is not accurate. If it can be repeated than the data can prove a very valid point. 

Answer 2

They need to be repeatable to prove that results from the expirement are viable, that it didn't just happen because of a series of things outside of the scientists control. Repetition just makes the expirement seem more credible.