A step stool can serve multiple purposes. As well as being used to reach high places, it is a stool. Some are designed to look nice enough to be used as regular furnishings or furniture instead of being stored away in between uses. Also, a small step stool can be convenient for people who don't like to sit on the floor while doing activities that require being lower down than a normal chair. Many step stools will also provide extra, temporary seating. Some children prefer sitting on a step stool, and some step stools have a nice padded top level which makes them a great alternative to a highchair for children who are old enough to sit on a chair, but maybe a bit too short to reach the table where the family is eating comfortably.
As with ladders, it is important to make sure the stool is stable before climbing up. You shouldn't reach out too far because it can create an imbalance.