Here's why. Just because you understand something doesn't necessarily mean everyone else does. So it may bear repeating if, after asking if everyone understood it, someone says “no.” Sometimes people are too shy to admit they don't understand, so repeating a message is the conservative thing to do. Further, I find that as important as I may think my message is, the minds of the so-called listeners may be wandering. Or the listener may be more focused on his or her retort than taking in what is being spoken.
Another possibility: the listener may have misheard the message. This is increasingly common as more and more people are “quasi-listening”: tinkering with their smart phones when they should have a laser-like focus on the speaker.
Early in my career, when I first got into management, someone told me to follow this rule: your top subordinate needs to be told a message only once. Your second most competent subordinate has to be told the same thing twice. Your least competent subordinate may need to hear the same message three or four times. I believe it is important for all managers to keep this easy rule of thumb in mind. It may not always be true, but it raises a consciousness that what we say is not necessarily heard. And if you need to have the task done right, there is no risk in over-communicating.
I'd advise you use other channels for critical communications, following oral delivery, to ensure the message is clearly received. Written memos. Email memos. PowerPoint. Texting. And the list goes on.
In summary, over-communicating in today's environment — with all the distractions afoot — may become the rule rather than the exception.