Answer: Polar curves do not need to be always bounded, suppose you are graphing something like a half-circle, the equation can be expressed as:
C(θ) = acos(θ) and 0 < θ < π
Another example may be if you want to describe a spiral, where the function may be thought as
C(r,θ) = r(θ)*cos(θ)
where r(θ) = A*θ, so when the angle increases, also the radius, and then the curve is not bounded.