Causeways are what make Vice City work as a city. Without causeways, the strip and mainland are two distinct areas. With causeways connecting them, the city flows how a Miami-coded coastal city should.
Note that we're just discussing the causeway crossings here. See the iconic bridges piece for the wider bridge map.
What the causeway frames show
A long, low road across still water. Multi-lane traffic. A row of lights along the centre median. Yachts on one side. A shoreline on the other. Trailer 2 has daytime and dusk versions of similar shots.
The causeway in Trailer 2 looks like a road people would actually use and drive to work on - not a sightseeing route. That's a different atmosphere from a tourist drawbridge, and it's probably what Rockstar is going for.
How many causeways are there?
The trailer aerials point to at least two, plausibly three - matching the actual Miami configuration of the MacArthur, Venetian and Julia Tuttle Causeways. The exact number is unknown.
Why the causeway matters mechanically
Causeways are choke points. If the strip is on the barrier island and downtown is on the mainland, every drive between those two places will involve crossing water. Police chases that originate on the strip and try to flee will have to cross water. Heists that originate downtown and try to escape to the strip will have to cross water. Causeways are going to feel meaningful in a way that very few GTA roads do.