When a shot is fired, the elastic properties of the barreled action let it flex in all directions from the energy transmitted to it beginning when the firing pin starts forward and ending when the bullet and gas leave the barrel. These flexing motions move the muzzle at right angles to the bore's axis. If the muzzle isn't in the same place for each shot relative to where it's aimed, each bullet will go to a different place. That's what causes some rifles to put all bullets into a half-inch or smaller hole at 100 yards and others cover an area about the size of the palm of your hand.
A receiver that's just put in a routed-out stock with enough clearance to handle their dimensional differences, plus make factory assembly easy, will typically clamp downward at random points as the stock screws are tightened. When the rifle is fired, the flexing and vibrations from firing that shot move the receiver a couple of thousandths of an inch around in its stock; after each shot, the receiver is in a very slightly different place. And for each shot, the receiver starts from a slightly different position with different stresses on it from the new set of contact points with the stock. Although these differences are small, they're enough. So, for each shot, the movement of the receiver changes just a bit and the barrel goes along with it. Which explains why bullets fired from plain bedded receivers tend to shoot groups of one to two MOA. Some rifles will shoot better than others due to their bedding being a bit different from rifle to rifle; some receivers just fit better.
If the receiver is moulded into a pool of epoxy about 1/8th inch thick all the way around it and that epoxy is both chemically and mechanically bonded very strong to the stock, that receiver just doesn't have any place to go after each shot. With the best epoxies used for bedding, the receiver will fit perfectly to less than a couple ten-thousandths of an inch. You just can't cut wood and fit metal to it that precice. You can't even cut a metal bedding block to fit that close. And that receiver will stay there from shot to shot forever, or at least while several barrels are worn out.
The only problem with this is that with heavier recoiling rifles, the amount of torque applied to the receiver by the bullet going down the rifled barrel can eventually compress some of the wood or synthetic stock. When this happens, the receiver is no longer a perfect fit in the epoxy; it starts being at a slightly different place for each shot. This is a typical thing with round receivers and bullets weighing more than about 160 grains. Flat-bottomed receivers typically don't have this problem; they stay in place very well.
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