Jono: I think once upon a time somebody thought it was. People thought it was a good idea to put the keywords they want to rank for in bold because Google would  “recognize” those and deem them important. I don't think it ever worked like that. Somehow, there are still people doing it. Maybe it correlates as being a _bad_ ranking factor. If you're bolding your keywords instead of thinking about how to make this text good and readable, you're probably making things worse.