Hey Mary, glad to see you posting! The answer would be if you are trying to represent a 2-beat note at the end of the measure (in beat 4). If you saw a half note in beat 4 of a measure in 4/4 time, it would not work because you can’t have 5 beats in a measure. Having a tie from beat four of the measure to beat one of the next measure would create the same sound as a half note, but be technically be correct in the meter. So it’s basically used when you you need to keep a measure at the correct amount of beats. I know you aren’t a composer, but that is what they are thinking when they create it!