Do companies have their own instance of Ariba?

I have some basic questions on SAP Ariba. From what I understand, Ariba network has million of suppliers and buyers can search for their requirements on this Ariba network. So looks like there would be one global Ariba cloud based hosted website, which all customers(both buyers and suppliers) would use.

But, my Ariba knowledgeable colleague was saying that companies have their own instance of Ariba and in this instance they feed their suppliers, but this sounds very counter-intuitive. If companies manage their own suppliers, then they are not using the millions of suppliers who are listed with Ariba network.

Why would company want to maintain their own Ariba instance?

Is it for buying processes streamlining feature that Ariba provides, and they are not interested in dealing with all the suppliers registered with Ariba network?

If I should be asking this question on some other stack exchange site, please let me know.

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