It overrides whatever theme the blog owner has specified, in order to format the text and images on the blog to make it look better on a mobile device.
Weirdly it also includes advertisements, the content of which the blog owner doesn’t seem to have any control over. They’re randomly inserted in by WordPress/Google Ads, based on the content of the blog entry.
Earlier today a reader who looked at my blog through their iphone sent me this image.
It appears that Google ads is not without a sense of humour.