I guess the short answer is that cream and sugar in coffee tastes really good; it's delicious. But there are practical, circumstantial, and historical reasons for it, too. Even die hard coffee professionals I know like to fix up “dessert”, as they often call it, and have a sweet, heavy cream-laced coffee once in a while.
From a practical or circumstantial standpoint, adding cream and sugar to your coffee is a great way to balance a coffee that is roasted very dark or has any ‘off’ flavours. In desperate situations and before our instant Brazilian coffee was available, I'd "dress up" some gas station coffee with cream and sugar on a road trip to make it bearable.
While I can't speak for others, I can share a few reasons why I occasionally modify my filter coffee. For instance, I enjoy pairing a darker roast coffee with a savoury hot breakfast and find that heavy cream complements the bold flavours better than black coffee. I find that cream and sugar create a well-balanced cup when I want to savour our darkest roast, the Strongback Blend, and our most popular dark roast, the West Coast Trail Blend. Cream and sugar is not just meant for dark roast, though. A lot of our cafe customers enjoy it with our most popular medium roast, the Fernwood Blend, and of course, our espresso blends.
Historically, the first people to consume coffee drank it very strong and it was brewed to be quite bitter. In the coffee houses of Europe people started adding sugar to balance the flavour which didn’t suit their palette. They were accustomed to the lighter taste of tea. A cube of sugar mellowed out the coffee and because sugar was very expensive, it served as a bit of a status symbol. With industrialization, sugar and dairy became much more accessible and sweet milky coffee became an every day beverage.
But at the end of the day the most important thing is to remember that we should be approaching any tasting with inclusivity and accessibility in mind. It’s important to enjoy coffee and trust your own preferences rather than conforming to expectations. Throw out the rules and enjoy what you enjoy.