Why should I as a Plankton power down?
If you are using Bid and Promotions Bots then you spend SBD for Votes so i power down and convert my steem back into SBD to then buy Votes, to then get more SBD.
After claiming the reward of my posts and then after the 7 days of the power down ( 1st Reward) i restart the power down with the new higher amount of steempower and that i repeat.
Also most of the time you should be able to claim more post reward than you can power down per week. But if you power down steem won't undelegate that much because then you have let's say you get 1 SP from the post so then you have 15.75SP and 0,25 STEEM with the Power down. So steem will then only take 0,75SP of the delegation ( so you have 15 SP again ) but without Power Down you would then have 16 SP and steem will subtract 1 SP of the delegation, and you do not get any steem.
Conclusion: Powering down and converting it to SBD is very usefull when using PROMOTION OR BID Bots. Also steem won't subtract a high amount of the delegation, because with powering down you do not have 15SP and so for a 1SP payout if you have powered down to 14SP then with the payout you will have 15SP and steem wont subtract anything of the delegation.