The main reason why the human brain tends to utilize heuristics is because our brain has a natural need for closure which is obtained through a diverse forms of problem-solving skills. Moreover, the natural tendency of the human brain is to make immediate connections, or building schema, as a survival mechanism that will help to collect information instantly to make decisions, particularly last-minute decisions.
Most importantly emotions, in the form of affect, play an enormous role in heuristics. This is because most of our long term memory is built and kept through motivational process that can only be activated by emotional triggers. This is what produces our fears for things that we associate with scary situations in a way that imitates classical conditioning.
To put it more specifically, let's explore this example: a gecko jumps on your arm, you scream, and now you create a perennial dislike and fear of geckos. Every time you think about geckos you will not help but thinking about "that time when a gecko jumped on your arm". It was the emotional part of your brain (frontal cortex) that created the affective (negative) connection to the gecko and, as a result, you will keep that memory in the long term memory.
What happens next? That every time you walk a path that is similar to the one where that one gecko was (say, a garden, a forest, or somewhere where geckos may live) you will use the memory of the jumping gecko to, a) never walk there again, b) walk with precaution, and c) look out for geckos. All of those rules of thumb are the heuristics that your brain utilizes to make the final decision on whether to walk the same path or not. Although you are not analyzing the situation more in-depth, you are using the most salient emotional aspects of the memory to make your choice. This is what humans traditionally do as a cognitive defense.