A common use of files is saving something between runs of a program, like a high score. Read it if the file exists, use a default if not, and write it back when it changes:

def load_high_score():
    try:
        with open("highscore.txt") as f:
            return int(f.read())
    except (FileNotFoundError, ValueError):
        return 0

def save_high_score(score):
    with open("highscore.txt", "w") as f:
        f.write(str(score))

best = load_high_score()
print("Best so far:", best)
save_high_score(best + 10)
print("Saved:", load_high_score())

Run it a few times: the score keeps growing, because the file survives between runs.