A procurement solution is part of a larger picture. Even in the simplest approach, you will typically want your eProcurement system to communicate with your General Ledger system for accounting purposes.
In addition, you may want to integrate with payables, asset management, ERP, and other applications.
Your organization's integration needs should be an integral part of your analysis when selecting an eProcurement system. Make sure that your solutions provider has made accommodations for the eProcurement system to communicate with other systems.