The format was designed for efficient delivery of graphics over the Internet. The files are compressed and they do support incremental rendering through streaming. The format is simple to allow for a simple player. Files can be displayed without any dependency on external stuff. SWF files don't need fonts from the system, for example.
Files degrade gracefully on cheap hardware.
Files should be rendered very quickly and in a high quality. SWF files may include both sound and animation. They can also be sound-only files. The SWF file consists of frames which are ordered along a timeline. Therefore, these sound files are sometimes referred to as sound only timelines.