For a complex number $z = a+bi$, we say that its modulus is: $$|z|=\sqrt{a^2+b^2}$$
When we draw complex numbers in the Argand diagram, intuitively, this makes sense. But if we used a different projection for the diagram (i.e. a different metric for distance) then it wouldn't necessarily. Of course, complex numbers can also be written as:
$$z = re^{i\theta} = r(\cos\theta +i\sin\theta)$$
so an equivalent question could be, if this is what we define, why we define that:
$$|e^{i\theta}| = |\cos\theta + i\sin\theta| = 1$$
for all values of $\theta$, rather than just $\theta = n\pi$.
The answer may simply be that it is convenient to work with this definition. But is there a deeper reason? Are there any problems for which it is convenient to define things differently? And what would be the consequences if we did things differently?