Mechanism one: shared audiences
People who saw your streaming spot can be retargeted on social. People who visited your site from search can be excluded from prospecting so you stop paying to reach them twice. Customers can be excluded from acquisition offers entirely.
None of that is possible when three vendors each hold their own audiences in their own accounts. It is trivial when one team runs everything.
Mechanism two: managed frequency
Each channel optimizes to its own frequency target. Run four separately and your best prospects get hit far more than anyone intended while your reach stalls short of the market.
Managed together, frequency becomes a decision: enough exposures to be remembered, not so many that you are annoying people who already called.
Mechanism three: sequencing
Introduce with reach, follow with proof, close with an offer. A prospect who saw the streaming spot should not receive the same introductory message on social two days later — they should receive the next thing.
Sequencing is the difference between four channels shouting the same sentence and one campaign making an argument.
What it takes
One team with access to everything, one definition of a conversion, one budget that can move mid-month, and reporting that counts each outcome once.
That is an operating structure rather than a strategy, which is why it is usually the highest-return change available and the least discussed.