Buying outcomes
An agency, at its most common, delivers a result and takes the method with it when the relationship ends. The leads came, the ads ran, and the moment you stop paying, the engine stops, because the engine was never yours.
You were renting access to someone else's process, and the dashboard that explained your own business lived on their laptop.
Building capability
A system is the opposite arrangement. The point isn't a campaign, it's an asset that stays with you: a way of capturing leads, qualifying them, and following up that keeps producing whether or not any single vendor is in the room this month.
The knowledge of what works for your specific business accumulates on your side of the table instead of evaporating.
The test
This distinction sounds academic until a contract ends and a business discovers it learned nothing it can keep. The numbers went up for a while and then reset to zero, because nothing was built, only borrowed.
If the people running your marketing disappeared tomorrow, would you still understand how your own growth works? If the honest answer is no, you don't have a system. You have a subscription, and subscriptions end.
Results are something you can buy. Capability is something you have to build, and only one of them is still there next year.