In a nutshell:
Sans & Serif is a combination of Serif and Sans serif fonts, used in close proximity or headline, to create contrast and visual stand-out.
In the examples shown, it is mainly used as a graphic tool to contrast and italicise part of sentences – stressing a key point or word. Further than this, when used in a brand language, it helps to reflect the multifaceted nature of brands – showing a boldness while remaining sensitive. Furthermore, it also speaks to time – the sans serifs feeling more modern while the serif perhaps emphasising the heritage and roots of the brand. Converse and Nike have both started using Serif fonts in their brand language.
Also see Molten Type Typography