It's elaborated pretty well in The Verge:
The burgeoning trend of teasers within trailers exist purely to retain the viewer's attention in that exact moment.
Take for example, the trailer for Independence Day: Resurgence; the first 10 seconds are visually arresting — an alien ship turning Earth's atmosphere into flames, an upside-down skyscraper skewering a city like a giant dart. But the images, free of context, make no sense. They're simply loud and spectacular.
What follows this teaser is a (comparably) slower sales pitch, one that features the same footage, but takes time to explain what is happening in the scenes and why.
The teaser within the trailer speaks to a moment in which we have so many distractions and choices that marketers must sell us on giving a trailer three minutes of our time.
It's a era when we get so many hyped big budget film trailer every other day and we have chance to skip this one and play some other trailer, this teaser before trailer give chance to user to know what they signed up for in-advance to keep them engaged.