Programmers use a lot of symbols, often encoded with several characters. For the human brain, sequences like ->, <= or := are single logical tokens, even if they take two or three characters on the screen. Your eye spends a non-zero amount of energy to scan, parse and join multiple characters into a single logical one. Ideally, all programming languages should be designed with full-fledged Unicode symbols for operators, but that’s not the case yet.
Fira Code is an extension of the Fira Mono font containing a set of ligatures for common programming multi-character combinations. This is just a font rendering feature: underlying code remains ASCII-compatible. This helps to read and understand code faster. For some frequent sequences like .. or //, ligatures allow us to correct spacing.