You get to "talk" to the enire team perhaps a once a quarter, in some town-hall event where most people aren't really listening, and maybe send a mass email every now and then, which no-one reads. You get to talk to your immediate management team more often, and they talk to their reports, and so on, but of course whatever message you've had is greatly diluted, sometimes comically so, like in a game of telephone.
People are not robots, their understanding and communication is very ... creative. And context-dependent. And self-motivated. So that's not effective. Most people need to engage with the message you have - your priorities, changes in direction, specific goals - on a daily basis. Otherwise you are not really giving them any direction. And so you need some device to enable that. And OKRs are a part of the solution.