the only scenario i would consider is if :
a) you must redeem a reward within seven days (the maximum amount of time starwood will allow you to book and hold a points reward) and
b) within that time you do not foresee any points you've accrued posting to your account and
c) you don't live close enough to a *wood property to spend money and earn the points.
to me this is a pet peeve of the program: hyatt will hold a points pending reservation for as long as you need them to without your having to call back to rebook/reboot it every 7 days. they are also a lot more flexible if you are a small number of points short of the reward (they trust that once ensconced in their property you'll earn back the meager 70 or 60 points).
given that many of us hold *wood amexes and are decades long members of the loyalty program, this would be a nice accommodation.

if you have time to plan, there is no reason to purchase points if you can earn them in time for a reward.

but if it is last minute and you are 1K short just before redeeming, well, that would be the only scenario that makes sense, presuming the cost of the hotel is high enough in $$ to warrant redeeming points and not paying revenue.