The gezaira of Raba is that when Purim Sukkos or Rosh hashana fall out on shabbos we do not blow shofar or shake lulav or read the meggila.
Why? Because we are worried that he may not know what to do, and go to a Baki to find out what to do.
How can it be that just because of a worry that one may not know the halochos,
We are uprooting a biblical commandment it is no simple thing to just skip a mitzva deorraisa. So what is logic behind this gezaira that it is strong enogh to move a mitzva (every mitzva is needed in this world so how can we not do it because of a worry-cheshash)