What exactly does unlocking my SIM & CID do?

[dahui9]

I was reading about sim and cid unlocking and finding out tons of ways to do it and so on but my question is what exactly does unlocking my sim and cid do?

I've tried finding things on google and in the wiki but only finding programs to do so non that explain what I'm doing when i use the program.

I've been flashing everything on the wiki, I even made my own rom so I was just curious if it would do anything?

or if there is a post and i haven't found it yet explaining can someone direct me so we can get rid of a senseless post?

thanks

[Doom Tints]

If you have been flashing your own ROMs, you've already unlocked the CID.

As far as unlocking the SIM, that would allow you to switch carriers.