When implementing cryptographic primitives, you want to avoid branching on secret values. The reason why is that the CPU's branch predictor will attempt to predict the value that you're branching on, and thus something about the values that you're branching on gets revealed if you can see how long a CPU takes to run a task/perform a function.

This is more than just a theoretical issue. These channel attacks have been demonstrated in practice, even if the victim CPU is running across the internet.