Because it depends on how the forward were coded. I still recommend you to read about *args and **kwargs but let’s illustrate it with an example.
if you define your forward function like:
def forward(*inputs):
   # Here inputs is a list.
   x = convolution(inputs[0])
   y = convolution(inputs[1])
   c = x+y 
   return c

output = model(a,b) # here you pass directly a and b

without the starring you would have to pass a list
def forward(inputs):
   # Here inputs is a list.
   x = convolution(inputs[0])
   y = convolution(inputs[1])
   c = x+y 
   return c

output = model([a,b]) # here you pass a list with a and b as forward only expects a single input

But still, google *args and **kwargs cos you are trying to understand a python feature a pytorch rule