Here's why:
* They can distinguish meaning: Words like "tie" and "die" have different meanings solely because of the difference between "t" and "d".
* They occur in minimal pairs: Minimal pairs are words that differ by only one sound, and the difference changes the meaning. "tie" and "die" are a minimal pair.
* They have different places of articulation: "t" is an alveolar stop, meaning it's produced by stopping the airflow at the alveolar ridge (the bony ridge behind your teeth), while "d" is a voiced alveolar stop, produced in the same place but with vocal cord vibration.
Allophones are variations of the same phoneme that don't change the meaning of a word. For example, the "t" sound in "top" and the "t" sound in "stop" are allophones of the same phoneme /t/. They sound slightly different due to the surrounding sounds, but they don't change the meaning of the words.