Pyrotechnics

Explosions are an action film staple. Filmmakers continue to use pyrotechnics in production today because it simply looks extravagant and grasps viewers' attention. We continue to see real explosions in film, even well after CGI was invented, because of its authentic effect. Fire is a natural element whose physical characteristics are unpredictable and difficult to mimic with computer graphics. Hollywood chooses to use real, controlled explosions rather than create them in post-production because it's the only way to get a realistic explosion without all the tedious hassle of having to design an unpredictable explosion.
In this video the process of creating a basic explosion in post-production is explained. In the video, the fire was placed over the footage in a editing software called Adobe After Effects. Because this wasn't a real explosion, the flames don't move naturally with the environment the production is set in. The best way to achieve a realistic explosion is to film a small, controlled one in the same environment the production is being filmed in, and then chroma-keying in with the other footage like the guys in Film Riot explained. The process in making a small explosion that can be edited in After Effects can be shown here.