what's wrong with my code?

i'm a beginner so no roasting pls :)

my initial code for that task was:

def left_join(phrases: tuple[str]) -> str:
    # your code here

    Joined = (",".join(phrases))
    Joined.replace("right","left")

    return Joined

it seems like the only thing i had to do is to assign Joined into another string and then return that new string. my initial code returns a wrong answer while this code :

def left_join(phrases: tuple[str]) -> str:
    # your code here

    Joined = (",".join(phrases))
    result = Joined.replace("right","left")

    return result

results in a right answer. how come? why is like that?