if tuning the ILS you tune localizer freq which is between 108 MHz to 111.975 MHz. why the glide slope freq which operates in the 328.6–335.4 MHz band is tuned simultaneously?
A glideslope signal is useless without a localizer signal (i.e. if the localizer is out of service, the entire ILS is out of service). There is never a case when a pilot would be expected or required to tune a glideslope frequency unless they were tuning a localizer frequency at the same time. To make things easier, and to reduce the possibility for user error (the pilot tuning the wrong GS frequency for the desired LOC), the two frequencies are used in known paired sets and the airborne equipment will tune the correct GS frequency associated with the LOC frequency.
This is also how DME works; DME stations operate in the 960–1215 MHz band, but the pilot simply tunes the desired paired VOR frequency without having to enter the DME frequency as well. However in this case there are standalone DME stations not associated with VORs. These stations are still referred to in aeronautical publications by the associated VOR frequency.