Until the advent of CGI, filmmakers had to rely on practical effects and animation. Practical effects, such as those by Stan Winston in 1982's The Thing, were exemplary and actually better than CGI. The animation part, though, was most often not so good, as it was obviously fake and not the least convincing, as seen in the rocket launch scene from 1966's The Silencers.
The film industry is currently going though a period in that will be remembered in cinema history as the time when CGI began to seamlessly and convincingly blend with live action. Hollywood blockbusters, e.g., The Avengers, have become increasingly reliant on special visual effects over the past two decades, and recent advances in CGI technology have made feats in VFX work once considered impossible well within reach.
This site offers a sampling of 'before' and 'after' film stills to illustrate the last point.