Can be the difference between a solid OHKO, 2HKO or 3HKO.
The stats will differ by up to 31 points. Which can make all the difference. It'll also dictate how fast you can be and so what you can outspeed with different Speed EV investments and/or scarf.
For defence Ev's - see top sentence but now apply it to how easily you could be killed. The lower they are, the more likely it is. But some pokemon can't stand up to others even when perfect. For attack Ev's - See top sentence.
In-game. Don't worry. They never matter.
Black FC: 2666-8397-4718 Author of the Starfox Story on the Starfox Assault board. Check it out! Don't Fear the Reaper