Because it makes a difference if your attack leaves the opponent with five HP.....

Or zero.

Because it makes a difference if you can absorb four hits....

Or five.

Because it makes a difference if you get killed by a one hit KO.....

Or withstand it.

Because it makes a difference if you get killed by a faster opponent......

Or outrun him.

The choice is yours. To get the most out of your Pokemon(and if you still don't know what EV Training is), a simple guide is at hand.