Windowing modulates the input signal so that the spectral leakage is evened out (spreading on-bucket signals more and off- bucket signals less). Thus, windowing reduces the amplitude of the samples at the beginning and end of the window, altering leakage.
Journal of Object Technology
Lyon, Douglas A., "The Discrete Fourier Transform, Part 4: Spectral Leakage" (2009). Engineering Faculty Publications. 73.
Douglas Lyon, “The Discrete Fourier Transform, Part 4: Spectral Leakage”, Journal of Object Technology, Volume 8, no. 7 (November 2009), pp. 23-34