How and why are liberals committed to constitutionalism?

Liberals are committed to constitutionalism due to their belief that in order to prevent a government from becoming overly powerful, there needs to be some kind of check to that power. Constitutions provide a reference point for governments to adhere to and for people to use when to see if their government oversteps their remit.
Liberals believe that constitutions are necessary because of the idea that power is corrupting and there needs to be some kind of provision to redress imbalances, lest powerful governments infringe on individual liberties.