Organisation-wide purchasing is often the responsibility of the procurement team with the business reliant on them to source the right goods and services at the best price. But different departments within an organisation will often have very specific and specialist requirements and there are significant merits in empowering selected personnel to source exactly what they need, when they need it.
When multiple vendors compete for your business, you can expect to see aggressive pricing. But with your existing suppliers, competitive sourcing is not always an option, and without that you may be paying an above-market premium.