Even if the area under your stairs is small, there are plenty of ideas for utilising this space and making the most of it.
For a small area, consider having a series of large slide out drawers which can store items such as warm outdoor clothes like scarves, hats and gloves or footwear. Or you could construct a series of book shelves which fit snugly under the stairs. If you do not need the space for storage then you could make a cosy, reading corner with a banquette seat, plenty of cushions and overhead lighting. If you have pets, you may want to make this area into an indoor kennel for your dog.
If the space under your stairs is a little more spacious, at least big enough to stand under, then you could consider having a downstairs toilet and washbasin in the space. Perhaps the space is big enough to turn it into a small office area where you can keep all the household filing in one place.
With a little ingenuity, you could turn your wasted, difficult-to reach-into, space under the stairs into something far more useful and highly practical.