As it turns out, you actually don't! Click through to see why...
As meat cooks, the muscle fibers start to firm up and water gets pushed out. This moisture moves outward toward the surface of the meat, where some of it eventually evaporates.
When you take your roast out of the oven, the moisture still inside needs some time to redistribute back through the meat. If you cut into it right away, the liquid will actually pool out and your beautiful roast will end up very dry. By letting it rest, the moisture is re-absorbed and your meat will be tender and juicy.
There's also a second reason why you want to let it rest. A large piece of meat will actually continue to cook for a few minutes after you take it out of the oven. This is called carry-over cooking and is why many recipes tell you to take meat out of the oven a little before the meat is done cooking.
Once you take it from the oven, tent the meat in foil to keep the meat warm once the internal temperature peaks. The amount of resting time for a particular cut of meat or cooking technique will vary, though it's usually between 10 and 20 minutes. Your recipe will be your best reference for this.
So hold back on cutting into your roast right away, let it hang out on your counter while you whip up a fabulous gravy or put the finishing touches on a side dish, and then slice into it just before serving!