Why does a pattern call for this step? Sewing Machines Parts and Sales in Los Angeles

I have been sewing for a very long time, but haven’t done it in the last sis or eight years. I decided to start up again, but I came across a step I have never seen.

I am making a dress and I have sewn two pieces of contrasting fabrics together to complete the length of the dress. Now the pattern is asking me to iron open the seam and sew a piece of single fold Biased tape across the seam on the inside.

Any idea what this is supposed to accomplish?

Please and thank you.