Know Your Customers.
Chatting to a colleague yesterday, I was surprised to learn how often marketers still fail to discover how their category users decide on their purchases, let alone why.
The primary concern of these functionaries remains the sourcing of messages/proclamations designed to change people's behaviour. Well, good luck with that. Behaviour change is very, very difficult.
It's surely much simpler to seek to understand existing behaviour and then position your product/service in that vicinity either by devising new products/services or by reframing the user behaviour in terms of your existing products/services.
Marketers often claim to know who their customers are, be that in demographic or, more usefully, related behavioural terms. I prefer a more obvious definition. Your customers are those people who decide that your product/service meets one or more of their needs/wants in a better way than any existing alternative.
If you don't know why, then you don't know who your customers are and you're probably doomed.