The modern fighting game doesn't exist without a fireball, or a projectile that fulfils the same role. But rather than expanding fighting games beyond blocks, punches and kicks, it also gave 2D fighters more ways to explore space and strategy.
The importance of the fireball was chronicled by Core-A Gaming lately, covering not just how the introduction of the fireball expanded the arena of 2D fighters, but how it expanded the psychological game.
There's also some great insights from Seth Killian (who was working on the super interesting Rising Thunder until Riot bought his studio out), timely quotes from Daigo Umehara, and Tekken series producer Katsuhiro Harada.
If you've got more time to kill today, Core-A's older video on how to pick characters is also a great instruction in play styles, the perception of strength in a meta and the value of picking characters that you're physically comfortable with.