Why we need fourier transform of periodic signal although we have fourier series for periodic signal?
If you take the Fourier transform of a periodic signal, you'll arrive at the same result that you would if you had computed the signal's Fourier series. Its spectrum will consist of discrete lines that correspond directly with the terms in the Fourier series. As an example, note the following Fourier transform pair:

$$
\cos(\omega_0 t) \Leftrightarrow \pi \left(\delta(\omega - \omega_0) + \delta(\omega+\omega_0)\right)
$$

If you computed the complex Fourier series of $\cos(\omega_0 t)$, you would find exactly two nonzero coefficients: those that correspond to frequencies $\omega_0$ and $-\omega_0$.