Difference in effective length between states is too big. Transition preview will be disabled.

I get this warning when I click on the transition, it has exit time of 0.5, no parameters. I figured out that I get this warning only when the exit time is lower than my animation length. But why do I get this warning? Why's it bad that the exit time is lower than anim length, I don't need my animation to play over again, should I set exit time exactly as long as the animation is?