What does the rocket actually do?

[Grossout]

The goal is to launch the rocket but why ? Engineer doesn't even leave with it.

[Fel]

It does two things.
The first one is bring up the "win" screen for the first rocket of the map, being the end point of many achievements as well.

The second is giving you space science if you launch with a satellite, allowing you to research the infinite technologies, usually as a part of a mega-factory.

EDIT: As for the reason in terms of "lore" of sorts, it would probably be along the lines of trying to contact whatever force the engineer was a part of, to be sent a rescue vessel.
Since we crashed with out spaceship, going into space in a rocket wouldn't really help all that much since you would need something of a much higher technology to be able to go home.

[Hurkyl]

FWIW, AFAIK they were planning space content way back i early access -- they just decided they were at a point they could release a finished game with the rocket launch as the story ending, rather than keep the game in early access for a zillion more years.

[OldWhovian]

If you put your car in the cargo slot and press enter next to the rocket you can ride the rocket

[Vapor Dosh]

Originally posted by OldWhovian:

If you put your car in the cargo slot and press enter next to the rocket you can ride the rocket

Holy crap, I never knew that and I had to test it on an older save, it works!