Brambles spread by throwing out long,thorny stems which reach down to the ground and root there. These can be about 6' long, so the bramble patch spreads really quite quickly. I agree that these patches are wonderful havens for wildlife, providing food, shelter, nesting sites, nectar and pollen, not to mention fruit for picking for us.
Apart from the spreading bit, they are predators too, of humans and sheep
. Those long stems wrap themselves around human legs and hang on, but it's more serious for sheep. They become entangled in thorny stems and simply stand there til they die if no-one finds them. The bramble meanwhile has got itself a couple of years worth of fertility as the carcase rots.
If it were my patch (and I do have some) I would leave it be, but keep it trimmed around the edges. Whenever you see those long stems beginning to root into the ground, dig them up. That way you will limit the spread. It's worth fencing off the main patch if you are going to have sheep or small children in the paddock.
The wildlife will love you