The way I understand the question, this would be because political majorities would in principle be able to do anything, since they have the political power (even immoral things and things which are not compatible with constitution).
Judges can make sure that the laws introduced are not illegal (that is: are not at odds with the constitution) and in this way they keep the country safe from at least breaking the constitution.
Also, they can make sure that the political majorities don't abuse their power in other ways, such as imprison people without a trial. People then "prosecuted" by the political majority can go to a court and demand justice
