Eg, if I see FOR I = 1 TO 10 : stuff : NEXT I then I know understand stuff is meant to happen 10 times. If I see I = 1 : DO WHILE I > 10 : stuff : I = I + 1 : LOOP then I have to think a bit harder to arrive at the same understanding.
It is worse because usually the condition is based on "calculations", which you must put before and duplicate them within the loop.
This is worse than readability, in my opinion.