after filtering the power line noise, electrode movement artifacts, and muscle and eye-blink noise , why we need filter the EEG signal for suppressing frequencies over 30 Hz ?
human brain produce mainly 5 different frequency bands, Gamma is one of these bands in range of (30 - 100) Hz. and some studies found that Gamma band can be more accurate in classify some mental task.
that is why we can not just consider it as a noise, here is a paper where they found Gamma band gave a higher classification percentage of a metal task.