He probably means in case he doesn't have enough brews to use ironskin anymore, and want to use his only brew at that time to purify instead of delaying even more damage.
I guess it's fine as long as you don't get killed by the next hits, but in my opinion you should better learn the fights to know when to spare the brews for purify.
Either that or, you still don't have any idea how stagger works @shade3891.