Generally, worldbuilding creates worlds that are at least somewhat like our own. Not always, but for the most part. They have sunlight like our own, climates and weather like our own, etc. That's not a bad thing; it often provides a somewhat neutral backdrop for the exploration of a culture or cultures. However, not all backdrops have to be neutral, and sometimes it's worth it to stretch those worldbuilding brain cells and come up with something... extreme.
Extreme environments can include any number of things, from worlds in which volcanic activity has continued long enough for sentient species to evolve within them, to worlds with far more sunlight than our own, to worlds scoured by high winds, constant winters or trembling ground. Under these types of conditions plants, animals and sentients would all have to evolve differently. They would be much less like the life forms we see every day.
Take a volcanic world, for example. Even assuming the dust and smoke would block out a lot of the light, perhaps even all of it, that doesn't mean that the world has to be lifeless. There are plenty of life forms in our own world which don't require sunlight. Chemosynthetic life derives energy through chemical means and lives in places that would be toxic to any other form of life. From the hot vents to the cold seeps, these life forms feed on hydrogen sulfide or methane that has dissolved in the water. It's even quite possible that life on Earth evolved from such life forms and extremophile bacteria.
What kind of life would a world like that have? How would it cope? Breathing smoke and chemical-filled air's probably rough, but perhaps life there has a means of filtering out the harmful elements? Or, at least, the elements harmful to them. You also have to consider where they get the chemicals that they convert to energy. Are they bound to certain locations -- their volcanoes -- or can they travel between them? How do they deal with the heat? And, most importantly, how does all of this translate onto the page? What does it mean for your characters and how to do you communicate something so different to the reader?
The volcanic example is probably one of the more extreme ones. There are less all-encompassing ways to create variant environments. Consider a planet that's tide locked, spinning at a rate that keeps one side dark and the other light. Or a planet with a thin atmosphere on which the wind constantly blows. Or a world with seasons that stretch on for centuries. While some of these ideas have been done, how you use them is what makes the world unique. These extreme environments present problems and problems force you to think of solutions. They fire up your creativity and make you consider your worldbuilding from new angles.
High winds keep plants small, close to the ground where they can find some shelter. Winds also produce storms, from tornadoes to sandstorms, and more depending on the other climatic features. Moisture levels and high/low pressure zones become a big consideration. Life forms may or may not be very different, depending on how they evolve to deal with the problems caused by a constant high wind. Remember that while vision is the dominant sense for humans (as a species), hearing is also a major consideration, and any species with hearing as a dominant sense is going to be different.
The point is to take the "problems" inherent to such a world and devise solutions, consider it from the angle of the life living there. Even if you never wind up using your extreme environments, they make excellent thought exercises.