1- fixed Q means that you will never be able to truly clean those annoying frequencies that can clutter your mix.
when you add (boost) you add frequencies, noise, distortion etc. That means that you add trash over the sound.
if you subtract (cut) then there is a filter that will subtract that frequency (ies) which mean it is not adding "trash" over your sound but actually cleans it out.
My friend the witch doctor he told me what to say
He told me
ooh eee ooh aah aah
ting tang walla walla bing bang