Every day, people across the planet ask themselves this question, myself included. When we are desperately searching for our glasses, wallet or keys, we might wish to have a photo­graphic memory, but the truth is we are designed to forget.

In fact, the majority of what we experience in a given day is likely to be forgotten in less than 24 hours. And that is a good thing. Think of all the passing encounters with people you will never see again, the times you spend waiting in a queue at the supermarket, and those awkward times when you find yourself looking at the floor while stuck in a crowded elevator. If our brains hoarded away every moment of every experience, we would never be able to find the information we need amid an ever-increasing pile of detritus.

So, if memory is not supposed to be a comprehensive archive of the past, what is the point of remembering at all? To answer this question, it helps to think about what it means to remember in the first place.

For more than 25 years, I have studied how we are able to recall past events, an ability known as “episodic memory”. Endel Tulving, the pioneering cognitive psychologist who coined the term, described episodic memory as the uniquely human capability for “mental time travel, roaming at will over what has happened as readily as over what might happen, independently of the physical laws that govern the universe”.

I first read this description of mental time travel when I was a graduate student, and I was deeply sceptical. Now, with the wisdom of age (something I will come back to later), I understand what he meant. When you recall a rich episodic memory, there is a palpable feeling of being transported back to a point in your past, a specific time and place. For instance, the smell of freshly baked pastries might remind you of having breakfast with your grandmother, or a song by the Stone Roses might conjure up your first kiss. Findings from my lab and others have shown that, at the moment of remembering, the brain appears to revert a bit to the state that it was in at the time, enabling us to relive these past experiences. This is why, if you have misplaced your keys, it can be helpful to put yourself, mentally, into the context where you last saw them. Getting in touch with the sights, sounds and thoughts from an earlier time period can be an effective way of accessing those memories.

Mental time travel isn’t just about reflecting on the past; it also orients us in the present. Consider what happens when you wake up, jet-lagged and confused, in a hotel room. After a second spent recapping the recent past, you can reassure yourself that you’re there on holiday, and then go back to sleep. People with Alzheimer’s disease cannot use episodic memory as a lifeline, and, as such, may feel frighteningly disoriented, floating in space and time.

Crucially, Tulving also proposed that mental time travel allows us to consider what might be coming round the corner. He came to this conclusion, in part, from getting to know Kent Cochrane, who had profound amnesia after a motorcycle accident. Surprisingly, in addition to his severe episodic memory deficits, Cochrane was also unable to contemplate the future. Tulving’s ideas have been substantiated by further research. In the UK, Demis Hassabis (who went on to co-found the AI company DeepMind) and Eleanor Maguire published studies of patients with an impoverished ability to imagine events, and others reported a stunning degree of overlap in the brain networks that are active during remembering and during imagining the future. Further studies have shown that episodic memory can allow us to construct alternative realities, to consider what might have happened if we’d made different choices in the past.

On average, episodic memory gets worse as we get older, and that is due, at least in part, to the strange developmental trajectory of the prefrontal cortex – an area of the brain that helps support episodic memory. In humans, the prefrontal cortex continues to develop throughout childhood and adolescence, only reaching maturity in young adults. Then it begins to decline in function, starting as early as your 30s (depressing, I know). Consequently, ageing is a bit like having a dysfunctional time machine that frequently sends us to the wrong place. For many years, I pondered why it is that the full extent of mental time travel is only available to young adults, with the rest of our lifespan spent making do with a suboptimal episodic memory.

But what is “optimal”, anyway? Perhaps episodic memory is functioning exactly as it should through our lifetime. Consider that for much of human history young adults would have needed to care and provide for their children. At this age, they would require a more focused episodic memory to keep track of the most current information about foraging or hunting sites, to differentiate between allies and rivals, and so on. Elders, in contrast, have traditionally played a different role, guiding and giving advice to younger generations. During this period, forming new episodic memories is less important than passing on the wisdom accrued from existing ones.

So, the next time you find yourself wondering “Why am I so forgetful?” perhaps you can take some comfort from the idea that your brain is probably doing just what it evolved to do.

• Professor Charan Ranganath is the author of Why We Remember: The Science of Memory and How It Shapes Us (Faber). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.

Further reading

Remember: The Science of Memory and the Art of Forgetting by Lisa Genova (Atlantic, £10.99)

The Memory Illusion: Remembering, Forgetting, and the Science of False Memory by Dr Julia Shaw (Cornerstone, £10.99)

Being You: A New Science of Consciousness by Prof Anil Seth (Faber, £10.99)