"Are consumers likely to be confused?" is the way lawyers explained the positive case to me. An "Apple" computer isn't likely to be bought by accident to be used as an ingredient in a fruit salad, but a "Sorny" TV might well be bought instead of a "Sony" model.
They also follow this up with "you have to aggressively defend trademarks just in case".
Which is why Apple Corps and Apple Computer had several legal battles over the years, especially once Apple (the computer company) started selling music
It doesn't explain why society thinks this is an effective and fair set of rules.