Said another way, you're making a filter. Zeroing out FFT bins is a brickwall filter. Brickwall filters have poor frequency and amplitude accuracy if you are

Said another way, you're making a filter. Zeroing out FFT bins is a brickwall filter. Brickwall filters have poor frequency and amplitude accuracy if you are trying to preserve the signal in the passband. A Flattop filter will give maximum amplitude accuracy, a gaussian filter will give good amplitude and excellent frequency accuracy. Other filter types can be implemented to require less computation for resource constrained systems under certain circumstances. Zeroing is the simplest filter to implement in code, but it's performance is essentially the worst from a signals point of view.