=> First question : What will happen if I choose 'decline payout' here?

Answer: You don't receive the rewards for that post. But the upvotes still impacts (increases) your reputation.

=> Second question : What does it mean by power down?
Answer: Meaning that you want to convert your Steem Power to liquid steem which is almost the same but readily available and can be transferred as when desired.

=> Final one : What benefits I will get by transferring to savings?
Answer: Nothing much. Your savings balance is STEEM and SBD tokens that are subject to 3 day withdraw waiting period. This is an extra security measure in case your account credentials are compromised.

I hope this helps! :)