This seems like a stupid question as most of you will know why a gas leak could be dangerous. But, this is always something that can use reiterating as it reminds you how necessary one of these detectors truly can be. For the most part, there are two primary reasons why this can be dangerous. First and foremost, and this is only alluding to gases that are combustible, they can cause house fires. Given the popularity of natural gas (as it is used for common household heating appliances), types of combustible gases can be leaked into your home.
Then, the other issue is with carbon monoxide poisoning or other types of poison that are direct effects of exposure to certain gases. While small amounts of carbon monoxide may not kill you (though large amounts most certainly can), it can lead to long-term health issues.