Why not just use Rectangular window for filtering?

This question has puzzled me a lot. I wanna know why people not use the ideal window: rectangular window for filtering. I realized people prefer using others like Hamming. What's the disadvantages of the rectangualr window?

Answer 1

[Ted Shultz]

More or less, the sudden start and stops can cause unexpected results, such as higher side lobes. Depending on your application, this can matter more than the small loss of signal.

More discussion of this here:

https://dsp.stackexchange.com/questions/25992/what-is-the-advantage-of-applying-a-hamming-window-for-mfcc-calculations