The thread sealant prevents air leaking past the threads when you crack the bleeders open (and the conical seal is opened up). If you are poping them open to bleed the brakes frequently the sealant gets tired and leaks can develop. I've never personally had problems with them bleeding, but then again I've usually followed the advice I've read that said to apply new sealant from time to time if they see decent useage. So, maybe it's not required and I'd get the same results without new sealant, but it's easy enough to do that I've never lost sleep over it.