Django determine if signals are necessary

Question

Signals are a very useful part of Django, But I am having a hard time figuring out when they are really necessary. Take the following ambiguous example:

class FooBar(models.Model):
    some_field = models.something(#...)

    def func(self):
        # do something
        signals.some_signal.send(#...)

so this will send a signal whenever the func method is called. But one could also do the following:

def func(self):
    # do something
    # do what was in the receiver function

Are there any classifications of when to use a signal, and when to just include it in the original location?

It seems as though most situations will work both ways. Are signals just more optimal?