The EV spread of 252/4/252 is so common because it maxes out two stats completely, and gives the remaining points to another stat. This is good for if you want to completely utilities the two main stats you'll be using.
For example, here's the base Leafeon I use:
Adamant or Jolly nature (minus sp attack, and ups either attack or speed)
- Swords dance
- Sunny day/Last Resort
- Leaf Blade
To make the most of him, I max out his speed (252) and attack (252). This allows him to move first, and potentially sweep in one turn.
Probably not the best example, but I hope this helps!