A symbol can express certain ideas much more effectively than text. Think of how well traffic signs are able to associate images with information (“merge left,” “school crossing,” and so forth) and, without a single word, compel you to take action (hopefully).
In the same way, brandmark logos (which consist only of a symbol or icon) can give your audience a clear representation of your company’s identity without the use of words or letters
This makes them very useful for global companies, since consumers in other countries can associate the logo design with an identity regardless of what languages they understand.
They’re also helpful when the name of your company is very long and doesn’t lend itself well to an abbreviated lettermark; a group called the “Pediatric Ophthalmology Organization” might prefer a brandmark that doesn’t draw attention to their unfortunate acronym.
However, a brandmark logo type can be a risky move. Since it’s only a symbol, a person looking at it won’t be able to see your company name (unless maybe you’re The Company Formerly Known as Prince). That means it might not be the best choice for a new startup or a smaller company that’s trying to get people more familiar with their brand.
Note that in that same study of the world’s top 100 brands, only 6% of them consisted solely of a symbol, suggesting that this type of logo works best for very high-profile companies that are influential enough to be widely recognized by a symbol alone.