Basically since your regular ID is only 5 characters, XXXXX, that means there's really only 99,999 different IDs. There are surely more than that many players.
the Secret ID basically makes it more likely that your ID is more unique, since the likelyhood of 10 numbers being exactly the same is very, very small. 9,999,999,999 different combinations, basically, if my simple math is right.
You can check your secret ID without hacking by using Hyper GTS. Google it.
Upload a pokemon to the Hyper GTS, and it's Rejected, but your computer saves a PKM file of it.
You can load that PKM file into PokeSAV. You can view your Secret ID this way.
Would you say what you mean, or mean what you say? Megaman Starforce 3 FC: 4340-7023-4737