Not an expert, but reasons I've discovered over my career: 1. Because words take up a significant amount of space. On mobile, icons improve the experience. 2. Words in other languages can destroy layouts, meaning customizing style for each language. For instance, potato in english is pomme de terre(apple of the earth) in Canadian French. Some other languages I've worked with are much worse. English can be very terse it turns out. 3. Drop down menus require more clicks than a toolbar. Word toolbars without images either read like a sentence and are confusing, or they are well padded and then will take even more space. 4. Pictures can be more accessible to many people (if done correctly) 5. Our brains react to icon faster than to words. We actually store words and icons differently as well. 6. Reading is hard, and expends more energy. Really. It has to do with the above.