P.org at plant level

Hello Experts,

If I assign one specific p.org to one specific plant(not assign the p.org with C.C),than its known as p.org at plant level.but when we create the P.O than we need to assign that P.org to C.C.  otherwise not able to create the P.O. so if we have to assign P.org with C.C so what is meaning of P.org at plant level.

I know this is very common question,however if  ill get my ans. it's very glad for me

Thanks in advance