And a lot of the hard work of making such an effects-heavy film believable lay with ILM. Therein lies the difference between Star Wars and something like Starcrash.
Before the SFX shots were inserted, I imagine the film looked a lot rougher & harder to visualize. Especially for the New Wave generation of filmmakers in Lucas’ circle of friends, whose films were usually set in contemporary society, and by & large didn’t rely on that sort of post-production effects work.