Why Fourier series if Fourier transform can be calculated for both periodic and aperiodic? While learning about Fourier Transform after Fourier Series, That we can calculate Fourier transform of periodic signals too. If we can take the Fourier transform of periodic signal too then my question is why we Fourier series if Fourier transform can be calculated for both periodic and aperiodic?
You can't take the Fourier transform of a periodic signal, the integral diverges for all multiples of the period.

This can be handled by the theory of distributions, but the Fourier series is a better fit.