C# new Mat("") BitmapConverter.ToMat(bitmap)

Mat mat;

bitmap.Save("1.jpg", ImageFormat.Jpeg);
mat = new Mat("1.jpg") ;

With the top one.

mat = BitmapConverter.ToMat(bitmap);

Are these two different?

I can use the mat on the top, but I can’t use the mat on the bottom. What is the difference between saving the bitmap as a file and reading it directly as Mat?