I've mentioned this before, but I think Star Trek III gets severely overlooked regarding the amount of world-building it established. Up to that point, we had seen VERY little of the universe Kirk and company inhabited. We take it for granted now, because we've seen 100's and 100's of hours of Trek since TSFS premiered, but at the time, this was the most expansive look at the Trek universe we had ever had.
Think about all the things that TSFS showed established, or expanded upon:
1. The Excelsior-class, which went on as a mainstay all the way through the rest of the films and the 24th-century television shows.
2. Earth Spacedock, which is now iconic.
3. The Klingon Bird-of-Prey, which became synonymous with the primary Trek villain for almost 2 decades.
4. The Oberth-class starship, which was a very unique Federation design.
5. Transwarp drive
6. Certain elements of Vulcan culture and society (Fal Tor Pan, Mt Seleya, the concept of a Katra, etc.)
7. Established the Klingons, in terms of refining / defining their behavior and appearance, as they would be known for virtually the rest of the life of the franchise
8. First depiction of a Federation Starship self-destruct
9. Considerable expansion of Sarek's character, who would become much more important to the franchise after this film
10. This would set up Kirk's continued hatred / distrust of Klingons, which became a key character trait in the next films.
Again, up to this point, we had nothing that even came close to this. The other two films did a fair amount of world-building, but not nearly as much as we saw in TSFS. The designs and other elements introduced in this movie had a significant impact on the look and feel of the franchise for the next 20 years.