What is the beneficient of taking the FFT of an ecg signal?
I'm not a doctor, I'm an engineer. However, I think I see the point of this.

An electrocardiograph is used as a diagnostic tool to assess the state of the heart and cardiovascular system. Several signals are measured, each of which can be used to make judgements about the health of the patient. Most of the signals measured in an ECG are periodic (in other words, they occur at a given frequency). The DFT/FFT is basically a way of analyzing the frequency content of a signal. So, it follows that by taking the DFT/FFT of an ECG, you can easily see those frequency components.

From the Wikipedia article on ECGs, here are some common components of an ECG and their associated frequencies:

P Wave: 0.08s, 12.5Hz

QRS Complex 0.09s, 11Hz

T Wave: 0.16s, 6.25Hz

I don't actually know the medical meaning of those features, but in any case, they're periodic, meaning you can analyze them using spectral analysis.