@blevley outstanding question thank you for asking this because other readers must certainly be wondering the same thing! Powering down allows payments of Steem Power into Steem once a week which can then be cashed out on an exchange. Witnesses have costs to maintain servers and costs often ascoiated with related projects which often are either not profitable or make a small profit. For example, my witness server, seed node, and backup server are about $450 a month in hosting. It would be reasonable for me to power down enough to at least pay the server costs.

Thus, powering down allows witnesses to change Steem Power rewards received into fiat currency. Ideally we want to keep as much Steem in Steem Power as possible to lower the market supply and raise the demand. One factor I consider in votes is whether witnesses are powering down because if witnesses consistently dump Steem Power earned on exchanges, then we also have a constant supply which is likely to equal lower prices. Witnesses with the most long term faith in Steem are likely to want to hold as much as possible because with a price of $10 coming in the near future, powering down and selling today could lead to missing 90% as opposed to holding. That said, we all need money to pay our bills and sometimes the witnesses that are all in on Steem also depend on the Steem Power rewards to pay the bills on projects which often do not have much of a return. Thus, it can be a delicate balance for witnesses especially in the top to keep the bills paid while also holding as much as possible!