I always thought the bolding was there for emphasis. And indeed, I found that if you read the dialogue with that in mind, you can get a sense of the emotion the characters are supposed to be conveying in their speech.
Including emphasis on the characters’ and places’ names, as well as the names of important things. Anybody ever notice how each time such a name is used in dialogue, it’s also bolded?
Picture a Superman cover, showing Superman in an angst-ridden pose (hand on head, look of anguish on his face). The Man of Steel is thinking something like the following in a thought bubble:
“Luthor has captured Lois and has her in his infernal Machine of Flaming Death! But there’s a meteor heading for Metropolis right now! … Who do I save? Who do I save?”
Oh, and the cover is complete with splash panel reading something like “The hardest choice Superman will ever make.”
Hey, this is kinda fun, and you know, the exclamation points add that special touch! Maybe I should post it, and then get some coffee while I think of more!