Are there any reasons or benefits for the very long process of reviewing grant proposal? I talked with some colleagues and it seems that the review process for most grants proposals (NSF, NIH,etc) are taking months (~6). This is probably not helpful for science and innovation. I understand that reviewers are busy people but papers submitted for a conference would usually not take more than 2 months to be reviewed. And some reviews that I read for conferences are even much better than grant proposal reviews. 

Are there any reasons or benefits that grant proposals are taking long time?
The processes for grants and papers are very different. Once the editor or programming committee has reached a decision, there’s not much left to do. 

Grants are much tougher. After the decision by the panel has been reached, there are several additional levels of bureaucratic approval that must be secured before a grant can be formally awarded. It’s these latter stages that lead to the extended time between submission and decision.