Can’t we just use our Facebook page or Google+?

The difference between starting a blog on your own website vs. a social network is that you have even less control of your content. Again, if you break terms of service on your social account, you would lose all of your content. You also have to abide by the ever changing landscape of social media..
The other thing you have to look at is the fact that you will lose all of the direct traffic potential of having content on your websites vs. having it all on a social network. Search engines would be more likely to index content on your competitor’s sites over your social networks. And if people can get your best content off of a social network, they will have no need to go to your website where you can increase conversions with custom calls to action.