Why is it important for scientists to defend their claims and conclusions to their peers

I think it's important because scientists need to be able to communicate. 
Think about why you write a lab report. When we write lab reports, we are effectively communicating how we did our experiment, our results, and what we think about those results. 

We need to have transparency and be able to replicate results in order to know if they are accurate to our hypothesis. Theories are not 100% proven. We only know what we are able to observe and all measurements have their limitations.