(Another) Question about EV training?

Question

Didn't know how to word the title.
I've noticed that in Competitive Play, people usually choose 2 stats to raise to the Max(Example: 200+ SA, 200+ Spe), and choose one stat to raise by 4. (Example: 4 Def)
I don't get this. Why not raise all the stats? Is there a rule that says you have to put that limit on the EV training?

Answer 1

Each Pokemon gets only 510 EVs to use on EVs of their choosing. A stat can have a max of 255 EVs to it, though people max it out at 252.

Why?? Rule of 4. 4 EVs = 1 Stat Point. 252 + 3 = 255. Those last 3 EVs would go to waste, so max is always 252.

252 + 252 = 504. That leaves 6 EVs left. Put 4 to any stat of your choosing. The last 2 are useless.

Answer 2

Ok so this is the best example I can think of.
Imagine a Shedinja
lets give it about 100 evs in each stat.
Now is it really worth it?
its Hp isn needed, its Def and SDef isnt needed and Satk cant be used.
Your wasting perfectly good evs that can be used in speed and attack to help it sweep

Basicly you only invest in stats you need.
Is there any point in giving shuckle speed evs or Satk evs?

So invest in the stats that actually need it.
Like Latios give it 252 SAtk and 252 Spe 4 Hp to use its full potentiol.