The primary reason for using the Hanning window is:

The primary reason for using the Hanning window is to reduce the impact of "leakage."

When performing spectral analysis on signals, particularly those that are not periodic within the measurement window, the Fourier transform can produce artifacts known as spectral leakage. This occurs when the signal includes frequency components that do not align with the discrete frequency bins of the FFT (Fast Fourier Transform). As a result, energy from one frequency can "leak" into adjacent bins, making it difficult to clearly identify the true frequency content of the signal.

The Hanning window, which is a type of tapering window, helps to mitigate this phenomenon by smoothly tapering the beginning and end of the signal. This reduces abrupt transitions that can cause leakage, leading to a clearer and more accurate representation of the frequency components. Consequently, using the Hanning window enhances the clarity of the frequency spectrum, allowing for better analysis and interpretation of the underlying vibrations in the data.