We had a briefing for a new campaign the other day. It was for a financial product, and it started with an explanation of how it worked. The briefing concluded with 'this is, of course, a very low interest category'.

This is a familiar phrase to any of us who work on finance brands or utilities – the classic 'low interest' categories. And it is often assumed these are 'rational' purchases, implying others are more emotional and 'high interest'. In our opinion, those assumptions are flawed.

We may spend our days understanding the fine details of our clients' products, monitoring closely the competition and advertising, but if we've any sense, we'll realise that ordinary people don't care much about any of that stuff. As far as they're concerned, almost all purchases are low interest.

People are interested in their families, friends, jobs, weekend plans, the news and celebrity gossip. Most of the time, they don't think about brands or products. Occasionally, they rouse themselves to consider which cheese to buy for a special dinner, or which tablet to buy for a teenage daughter's Christmas present. And everybody has a few purchases they are unusually picky about – perhaps an obsession with fine wine, or passion for knitwear. But most of us don't devote a huge amount of brain power to buying things. We just have too many other important things to think about.

And because we can't devote too much mental effort to shopping, we tend to rely on feelings, hunches and intuitions to guide our purchases, even in so-called 'rational' categories. Psychologists tell us that most buying decisions are either too complex or too trivial to be made analytically, so people tend to rely mostly on their emotions instead.

So, should we think of all categories as 'low interest' and 'emotional'? Not quite. It's more helpful to think in terms of different moments and states of mind. During the buying process, people oscillate between long periods of low interest, when they're guided by feelings, and short bursts of more intense interest, when rational thinking can play a role. The balance between and timing of these states is different for different people.

As marketers and communications experts, we need to be able to talk to people in both states. We need to create brands that stand out, even when people aren't interested in them. We need to be able to create an emotional halo that will influence people who don't care about our products, and we need to be able to provide rational arguments to convince the very few people who do.

And if we do our jobs well, we can make anything interesting. You would have thought that the dynamic steering systems of big trucks would be about as dull as it gets. But get Jean-Claude Van Damme doing the splits between two huge moving Volvo trucks just as the sun rises and we're all interested for a moment in the amazing stability and precision of Volvo. And if we were ever in the market to buy a big truck, our bet is that Volvo would be the first brand to come to mind.

Adding interest to things that people don't really care about is one of the most important functions of advertising. And if you look at the Super Bowl ads, you will see that the main way we do it is to tap into the things that people do care about: celebrities, small furry animals, young children, funny stories and so on. These work just as well for finance as they do for chocolate.

So next time someone says that a purchase is 'low interest', remember that that's what makes our job so interesting.