In other words, it prevents you from shooting yourself in the foot. In the old JSP days, it was very common to have JSP files sprinkled with Java code, which made refactoring much harder, since you had your code scattered.
If you prevent logic in templates by design (like mustache does), you will be obliged to put the logic elsewhere, so your templates will end up uncluttered.
Another advantage is that you are forced to think in terms of separation of concerns: your controller or logic code will have to do the data massaging before sending data to the UI. If you later switch your template for another (let's say you start using a different templating engine), the transition would be easy because you only had to implement UI details (since there's no logic on the template, remember).