Why WOULD republicans talk about Obama during the primaries?

Someone else asked about that but wouldn't that be like preaching to the choir? After all, democrats wont be voting in the Iowa caucus, republicans will. If you're a republican, and you watch Fox News then you hate President Obama anyway. I think the candidate that DOES run on their strengths during the primaries WILL be the nominee because you already have Fox News talking about President Obama everyday. But then again, when Trump started bashing the President, he got really popular so I don't know, maybe it will be who can say the most about the President, who knows?