Separation of powers is the machine built to make power fight power. It splits a government against itself, handing law-making, law-enforcing, and law-judging to rival branches so that no single hand can seize them all, on the wager that liberty survives only where power is forced to check power.
Imagine one person who writes the laws, enforces them, and judges anyone accused of breaking them. That person could do anything to anyone and call it legal. There would be no escape, because the same hand holds every part of the trap. The whole idea of separation of powers is to make that combination impossible. Split the work. Give law-making to one body, enforcement to another, and judgment to a third. Then set them against each other so that abusing power requires the agreement of rivals who have every reason to refuse. It is a government deliberately built to get in its own way.