@nn.compact usage in FLAX

Question

What is the use of @nn.compact in FLAX?

class Net(nn.Module):

@nn.compact
def __call__(self, x):
    self.n = 1
    return x

The same can be done by using :

class Net(nn.Module):
def setup(self):
    self.n = 1
def __call__(self, x):
    return x

What is the use of the decorative @nn.compact using explicitly in the code?