Why is higher stop band attenuation desirable in a LP filter?
A filter’s stopband specification depends on a signal chain specification. If the ADC sampling frequency (Fs) is much higher than signal bandwidth (BW) then the stopband attenuation of a simple RC lowpass filter can be adequate. For example, if the signal BW is 100kHz and Fs=10MSPS then a 100kHz first order RC lowpass filter provides -40dB attenuation at 10MHz.
If signal BW is 100kHz and Fs=1MSPS then a second order lowpass filter maybe required to provide -40dB at 1MHz. In general, the stopband attenuation required for any signal chain application must specify the maximum bandwidth that contains the total spectral energy of the input signal and includes unwanted signals at a higher frequency than the signal of interest.