Have you ever run your hand over fabric and noticed the colour change and how it reflects light? Fabrics such as velvet have a distinct nap or pile and when you embark on a project that is using fabrics with these reflective qualities, be sure you have enough fabric!
The pattern pieces for these types of fabrics will require you to place them all in the one direction, this will also use more fabric. If you were making a coat out of velvet, you would have the neck/shoulders, head of sleeves facing one end of the fabric, the the sleeve and coat hems at the other. This will ensure that when you sew the fabric that you have the pile direction traveling one-way and it will reflect the light the same.
The consequences of not cutting your pattern pieces in the same direction will result in the colour being reflected differently. In some instances this may be subtle, in others and depending on the colour of the fabric, it might be very obvious.
While this isn't a great example, the image shows that even with this brushed wool overcoat fabric, the difference in the pile or nap direction can be visual.