It doesn't matter if your Pokemon is Lv100 or not. Here, let me explain it to you this way:
You have a Lv50 Jolteon and I have a Lv50 Swellow. Your Pokemon has a base Speed of 130, which equates in a Spd stat of 135, while Swellow has a base of 125, equating too 130. Going head-to-head, you would feel pretty confident due to Jolteon's higher Spd stat, as it would go first and easily OHKO Swellow with Thunderbolt.
Now, 4 EV points in a particular stat increases that stat by 1 when it is Lv100. Since we are Lv50, it would take 8 EV points for that 1 point bump. Let's say a fed my Duggie 6 Carbos (because you always want your EVs divisable by 4), which gives it 60 EV points in Spd. At Lv50, that increases its Spd from 130 to 137. This time, going head-to-head my Swellow would give first.
That brings up another VERY important aspect that too many inexperienced trainers tend to overlook. A Pokemon's nature. In the example above, your Jolteon has a random nature. I randomly selected Quirky, which has no affect on Jolteon's Spd. My Swellow, however, has been bred for Jolly Nature (which I wouldn't do, but it works for this example), which boosts its Spd by 10%, increasing its Spd stat to 143. Once again, my Swellow goes first.
I know what you are thinking, Flying attacks are .5x effective against Electric-types. Jolteon will take some damage and then still OHKO Swellow with TBolt. That is where yet another overlooked aspect of battling comes into play - moveset. Too many inexperienced trainers for some inexplicable reason tend to give their Pokemon multiple moves of the same type (i.e. Swellow with Fly, Wing Attack, and Aerial Ace - don't laugh because I have seen it way too many times), severely limiting its ability to attack different types of Pokemon. In our above example, Swellow would be basically useless, as even Fly would barely do 25% damage to Jolteon. My Swellow however, has attacks of different types, which allows it to deal damage to far more types. Again, in the above example, I would definitely have Swellow learn Return, a 102 AP move (when it has max Happiness) which also enjoys a STAB increase, with it being the same type as Swellow. Now, instead of a ineffective move that only does 25% damage, Swellow hits Jolteon with a STAB move that can do up to 77% damage.
While that still enough to stop Jolteon from OHKO my Swellow, allow me to make a few changes. First, my Swellow would definitely have an Adamant Nature instead of Jolly, boosting its Atk stat by 10%. I would also definitely max out EVs in both Atk and Spd. Now, going head-to-head, that STAB Return attack will easily OHKO your Jolteon.
While EVs can be complicated and a pain to keep track of, you can see just how important they really can be.
