The Space Jockey is there as a form of world-building, while also establishing some foreshadowing. By including an unknown alien there, it shows that the in-universe situation is larger than the characters or audience is aware, that there is something more that's happened there. The specific point of having its suit show damage that was directed outward shows that something bad happened to it, and it was unexplained. Weapons usually attack from outside inward, but that wound suggests something happened from inside outward.
Why was the engineer fossilized in the chair?
To show the great deal of time that had passed since it had died. Something bad went down here, and no one came to retrieve/save them in that long time since it happened.
Why did the engineer's bio suit show an outward explosion?
It was because the Space Jockey got infected by a facehugger. Then a xenomorph grew inside it, and burst out of its chest. By showing this before the facehuggers were discovered, it foreshadowed the threat they posed to the humans.
This is very similar, thematically, to the start of the classic film The Thing when the Norwegians fly in and try to kill the dog. They both foreshadow a cycle of infiltration and eventually death.