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.